The wonderful thing about Whitworth spanners is that there are a lot less in a set. At the least you need 3/16, 1/4, 5/16 and 3/8. That will do about 90%. There are some larger sizes - 3/4, 11/16, 7/8 that cover things like the rear wheel nut, the fork top nuts and oil strainer (I may be wrong about the exact sizes of these 3).
Since Whitworth spanners are hardly used now days you can get them very cheap, e-bay is good and old British spanners (Brittool, KingDick, Williams Superslim) are fantastic quality and last forever so 2nd hand is fine. Brittool Whit sockets turn up at autojumbles, I got 6 for £4 a few weeks ago. A few sockets are useful - big end nuts, but beware, some aftermarket parts use AF nuts with Whit and BSF threads to save money so there are (for example) 1/2" AF 5/16 BSC nuts around, very annoying.
All the Whitworth spanners do 2 sizes as the nuts are smaller on the fine threads BSF and BSC. So a 1/4 Whit spanner does 5/16 BSF and BSC. This is very handy because you can look at the bolt or stud dia, if its coarse it will be the marked size, if its fine use one size smaller. This sounds odd but it soon makes sense.
Every once in a while BSA will bowl you a googlie such as the 25/32 AF nut on the crank. Bizarrely they seem to have made far more 25/32 AF spanners than the world could ever need so there are loads on e-bay. An added bonus is that Whit spanners fit BSP fittings so they will fit the petrol and oil pipe unions too.