Just touching the surface here...
With over 40% of our trade being within the single market of the EU, it could put businesses and jobs in jeopardy if we are no longer within the single market. Even if we renegotiate with the EU for equal trade status (in a similar way to Norway and Switzerland) we would still be subject to EU regulations - and most likely free movement of labour - but without being able to contribute to decision-making.
Leaving to EU would cause huge instability in the markets. Business likes stability and predictability. Brexiteers are unable to say what will happen if we leave. Renegotiating trade deals with the EU and other countries will be costly and very time-consuming.
Britain does well on inward investment, partly because of our lowish tax regimes, but also because we are in the single market and can be a base for foreign companies to do business across the EU. Would investors want the added red-tape of dealing with a UK separate from the EU?
We enjoy beneficial partnerships with the rest of the EU, the Erasmus scheme for student exchange being just one. Academics are hugely in favour of remaining in the EU. EU students also contribute greatly to our universities' income.
Free movement of labour benefits our economy and keep all kinds of services and businesses afloat; care of the elderly, the NHS, agriculture, teaching, IT, engineering, food processing, retail, hotels and catering etc, etc. EU migrants contribute more in taxes than they receive in in or out-of-work benefits. The large majority are young and productive; they had the initiative to move. We depend hugely on immigration to service our ageing population and will continue to do so. We have this in common with other European nations.
Rising population does put pressure on services. We need to make sure these are sufficient to cope with the immigration we need.
Brexiteers say we do not control our borders. This is only partly true. We control immigration from beyond the EU and we choose to allow free movement across the EU (with passport control into the UK since we are not in the Schengen agreement area). We choose free movement for the benefits it brings us and the freedom it allows our citizens to move to other EU countries to live and work (many thousands do, of course).
Larger nations like Britain and Germany do pay more in than they get out through the CAP and regional funds, but it is small sum per person. It does not take into account the profits which businesses make by trading in the EU. It is impossible to know precisely what our net gain or loss must be.
For me this is one of the key reasons to remain in this European project. European history has been characterised by wars between nation states. We have enjoyed an unprecedented era of peace and prosperity in western Europe which looks set to continue. To leave now would destabilise the EU and conceivably lead to a domino effect which could even see us return to the historical paradigm of inter-nation conflict. We should be immensely proud of this collective achievement.
The EU is imperfect, the Euro far from a proven benefit, Schengen under pressure owing to migration, but the bigger picture remains the same. We derive huge benefits from a single market. Wealth is shared and we all gain; as poorer nations in eastern Europe develop they become stronger markets for our goods and services.
The EU institutions are imperfect, but they do represent a democracy of sorts. The Council of Ministers where we have a say makes the major decisions, ratified by the European parliament which houses our elected MEPs. Our own British democracy is far from perfect.
You sometimes get the impression from the anti EU media that Brussels is always doing things "to us", as if we are not part of the decision-making process. We are in a club which works for the benefit of all. We are major players in decision-making.
If you live in a single economic market it is inevitable and desirable that you share rules so that businesses are on an equal footing. If a company makes a washing machine it needs to be saleable in 28 different nations. It makes no sense to have 28 different sets of regulations.
If we did not have EU red tape we would have to make up our own (to be acceptable to the EU and other nations). Red tape is a fact of life and the EU has taken steps to reduce it.
What will our farmers do if we leave the EU. Many (most?) would go under without CAP subsidies. Our government (taxpayers) would have to subsidise farming to maintain food supplies, as the Americans do. If we do not subsidise, prices from the farm and in the shops will rise and farmers will not be able to compete with their EU counterparts.
EU environmental laws affect us hugely and work to our benefit. CO2 and pollution does not recognises borders. Sensible environmental policy can only be made at an EU and international level. Thanks to EU-agreed rules we have cleaner cars, cleaner beaches, cleaner air, less polluted rivers and much more. EU targets help us move towards greener energy.
Although free movement makes our nation more multi-cultural and multi-lingual, this just changes our identity somewhat, as immigration always has done over the centuries. We should celebrate diversity and denounce xenophobia. When we hear unfamiliar languages it would be better if we were curious rather than uncomfortable. Why should anyone feel uncomfortable about? In any case, in or out of the EU, we shall continue to experience diversity.
The UK is a medium-sized power with the fifth or sixth largest economy in the world. Militarily and diplomatically we rank alongside France. As world leaders such as Barack Obama say, we exert more influence working along with the EU as one powerful bloc.
If we were to leave the EU it is quite possible that pressure will mount for another referendum for independence, one which may lead to the break-up of the UK. It's up to the Scots, but I would rather not see the UK split.
Gove, Johnson, Farage, Cummings, Lawson, Grayling, Tebbit, Putin, Trump et al. It's hard to identify with these people's views.