Polo Bikes

I have an unsplined 30 ring you can have for beers.

I have managed to find a replacement bike that I can get by the London Open, but it comes with 39/16 gearing. On my other bike i was running 32/20 which was around 41gear inches, so i need to sort something out tomorrow, i guess the easiest thing to do is change the chaiunring down at the front, which are BMX cranks I think. Any recomendations of somewhere good to go in London tomoz that could help me out with this? Prefferably not too far from Liverpool Street, as this is where i am coming into central london from, but not essencial