Eh no don't do it. The ISA's tax free-ness is amazing over time - no cap gains no divi tax nothing.
Once you're filling your ISA and using as much pension contribution as makes sense, then sure open an ordinary investing account. Interactive Brokers are great and yes absolutely they have cheap margin loans.... but you can only draw a certain amount against your investments, and if things drop hard you may suddenly find yourself margin called == selling low against your will. It is the sequence that is scary - you lose your job because of a recession and stocks drop because of a recession and suddenly you're selling off your stuff at a bad price.
Margin amplifies your returns - losses and gains. You want to have a solid chunk put away before looking at it. Like... £100k in your ISA and £150k in your pension, say? Something like that. THEN putting £50k, 100k into a normal account and using it to back £10, 20k of emergency money? Ok, that's all right, you won't get margin called with that and you have a lot of other stuff backing you up anyway.