Google 'Tower Records'(Picadilly) and see if they have what you are after in stock. The online search engine is excellent.

Think it is London's biggest record store, get out at Picadilly tube and you will not be able to miss it.

If they haven't.......do the same with HMV (Oxford st), just around the corner. And Virgin is worth a try too. All will tell you online what they have, to save the shoe leather.

Other than that, maybe someone else can suggest some smaller independent stores that may stock your choices?