In = status quo. Proven financial benefits for the country, some pressure from Poles/Romanians/etc. in the menial jobs sectors.
Out = Unpredictable financial consequences, more Indian/Pakistani/African immigrants, end of freedom of movement in the EU for Brits, barriers for British companies when expanding in the second largest economy on the planet.
Some more.
In.
+/- Transatlantic trade agreement
+/- has an aim of becoming a United States of Europe
+ has better consumer rights and is usually on the side of the people rather then business, and is so large that business has to fall in line
Out.
+ Undemocratic, it would fail its own criteria of joining the EU by not being a democracy
+ We were led into it under false pretences.
+ Our lack of commitment to the project is felt across the EU
+ As a result we have very little influence
+ I suspect the EU would be happier if we left.
+ I think we will both be happier if we rejoined in 20 - 40 years time ready to fully commit (if we wanted to)
+ Is going to expand and likely to have borders with Russia and the middle east in the not too distant future
+ Has been flooded with migrants that were invited by a few EU members but then want to spread around including Britain
+ Those migrants will hold EU passports in 5 years time and will gain the right to free movement
+ It has got too big too fast
+ Sometimes the European Court on Human Rights has overruled our courts and what the people of Britain want