Downing Street was constantly blocking developments, mostly to protect financial services and American interests. (They can veto many things, not only the budget)

The City of London with all its speculations and tax evasions has free access to EU financial markets but is only regulated (or rather protected) by the UK government*

A "EU army" for instance would mean synergy in weapon development and acquisition, but would diminish the overweight of the US and it's junior partner inside the NATO.

(It doesn't make much sense if Denmark and Belgium separately buy different fighter systems which need German territory as a training ground just 5 minutes after lift off.)

Finally multinational companies use the UK and Ireland as a base to enjoy access to the EU market while enjoying special tax deals.

I'm eager to see the "Celtic Tiger" trying to protect companies like Apple after the UK left. :)

PS: anyway Brexit will never happen, they will at best keep the status quo in a deal labeled "soft Brexit" or prolonged "transitional arrangement".

update

*) Brilliant news :)