Stay.
1) Security:
non-eu terrorists – they can’t get into UK as the UK has border control. They’d rather go to one of the EU countries after penetrating Greece or Turkey. As they say, to not get burgled you don’t need good security, just better than your neighbours.
Terrorists from within the EU – if they already have a criminal record they can’t enter UK as UK has border controls.
New terrorists from within the EU – even if Britain leaves, the necessity to negotiate a new trade deal will probably end up with free movement from the EU anyway.
2) Economy
Very complicated but apparently 9 out of ten economists say we’re better off staying.
3) Migration
This is a problem mainly for low-skilled workers who face competition from unskilled migrants happy with half the money. For the employers this is a benefit. Household help, builders etc would be much more expensive if we leave. There are also lots of doctors and nurses from the EU.
As far as changing the character of the UK is concerned, doesn’t bother me too much.
The only point is migration of Muslims, as it is recognised (even in todays politically correct world) that the anti-semitism ratio is far higher amongst Muslims.
There are lots of Yidden who benefit from the ability to move to the UK on the basis of a grandmothers Hungarian nationality.
4) Travel out of UK
Not just useful for holidays to not require a visa: I can’t help thinking that had the EU existed in the 1930s approx 6 million Jews would have been able to leave Germany and Poland etc. In todays increasingly right wing society I think that this point is valid if uncomfortable.
Hope this is all grammatically correct enough