FOR (things we gain from):
Being in the EU:
- gives us easy access to friends, relatives, colleagues, and employers, and they can visit us as well;
- means our voice (via the EU) can be heard coming from a major power block (pop 500M vs 60M);
- gives us access to EU-wide important projects (such as the EU GPS constellation, , ...) that we would not otherwise have access to, and that we have demonstrably benefited from in jobs and infrastructure;
- gives us access to investment funding in regions that currently lack it (due in part to the Westminster bubble);
- brings a degree of oversight of our own govt on issues like human and ecological rights which our Conservatives dislikes dealing with, and which IMO is necessary;
- helps keeps the peace with countries that we have in the past frequently been at war with;
Leaving the EU:
- gives us more freedom to interact with the world as we wish to (trade/policy/...);
- gives us more freedom to run the country as we wish to (e.g vat, subsidies, ...);
- costs us less money;
- means no risk of replacing sterling with euro;
- makes it harder to blame the EU for things that go badly;
AGAINST (things we lose from):
Being in the EU:
- costs us money (2016 ONS report: £18.9bn minus ~£9.4bn credits = £8.1bn per year) of £815bn per year budget, or just under 1%;
- means that people from the rest of the EU are able to come here (to work, play, ...) without our say-so, possibly doing jobs we might do ourselves;
- we are able to (would have to) set up trading relationships of our own (but it would take years);
- parliament would be sovereign - no other body could lawfully parliament what to do (with caveats);
- the EU defence cooperation might gain enough traction to become a real EU defence force, over time (is this good or bad?);
Leaving the EU:
- we would need visas for many more travel destinations;
- loss of access to EU financial markets (already happening);
- more difficult access to European security organisations;
- loss of scientific collaboration (which is currently very significant) with scientists in the EU and/or on EU projects;
- loss of (optimal) trade with EU partners (to WTO or some other...);
- trade requirements might force us to deal with countries with far lower health/safety standards;
- trade requirements may mean fewer world goods/services are adapted to UK reqs - (too small a market);
- loss of our contributions will badly impact poorer nations in the EU (via lower regional subsidies);
- loss of our military capability (France only other strong player) would make the EU temporarily weaker in defence/intelligence terms), which could influence the balance of world power;
- possible loss of Gibraltar to Spanish control;
- possible loss of Scotland to independence;
- possible loss of / civil war in Northern Ireland;
- a period of chaos, especially relating travel to/from/UK to anywhere else and which would affect goods as well as people;
- the EU might drop (would have no official need to keep) use of English in the official EU;