I use Uo-Chikyu (Hiroshima)(Made In Japan) and they're excellent. You can get a .010" to .046" set for 70 USD (approx 54GBP). That is without shipping & VAT, but I'd bet it still falls well below 120GBP.
Uo-Chikyu by Hiroshima Nut Files
With a 10-46 set you can comfortably do nuts to suit up to a .052 gauge set of strings (after learning proper technique).
Even if this TL kit is the only one you ever build, presumably you have other guitars, and with owning your own nut files, you can greatly improve their playability with proper nut action. Most factory nut set-ups are done on the conservative side to minimise possible buzz issues, not necessarily optimum playability.
Just my 2 cents. (that's 1.1 cents in GBP )