I think Universities are more lenient with grade boundaries if you are applying a year or more after you actually sit your A-Levels. (Feel free to clarify me on that if i'm wrong)
If you don't have that much money saved and you're considering Uni in London, a gap year to do some work might really benefit you next year when Uni rolls around. You'll be in a much safer boat and less likely to go into your overdraft - especially with that area of England being more expensive.
Also, if you're not 100% about International Relations, there's a chance that if you take it, you may drop out (and rack up needless debt) or switch course which can be complicated.