first fixie, first project. (stabilisers not inc.)

Looks like a nice bike. They look like 27" wheels to me so don't go getting 700c or it will look very gappy. Looks like a firm basis for a first fixie, as the front is just a single all you really need is a new rear 27" fixed wheel.
However, are you sure this frame is your size? That looks like a mighty high stem and saddle so will feel cramped and the handling will be twitchy. If you want to spend £100 then spend it getting something that fits you and is comfortable, only then look at making it a fixie and get some better parts.

From the clearances that are on it already, it won't look massively gappy with 700c wheels...there is only an inch in it at the mo, so just get some cheap fixed wheels from here...trying for a fixed 27 will be a right pain in the 'Arris'...

You may be able to keep the chain set if you have a 20 tooth fixed cog as well.

the frame looks ok and has the right drop outs, and after all...only cost £15...thats a bargain right there...

if you need any bits give me a shout I have some freecyle stuff...And...I have the spanner, tools etc Where abouts in London are you? You can use them, tho I'd rather they didn't get borrowed...

good luck and safe riding