Congratulations! Assuming, for now, that every part you put on this frame will say "British Made" on it somewhere. This won't be easy....

Okay, so... you need a Williams crank.

One of the first things you need to do is figure out what shifter fits the boss on the down tube. Probably Cyclo (but Simplex or Huret will probably fit as well).

You will need a pair of rims, 32h front, 40h rear. 27" would be correct, but 700c more convenient if you can find them.

GB Coureur brakes, or possibly Strata or something even rarer.

What fun, spending someone else's money in my mind....