Just a thought then, AKS (very highly regarded specialist) is in Bedfordshire, on route to Hinckley from London.
Speak to vRSAlex on here, he'll definitely be able to do the pre cat and pump, and no doubt sort you out the best possible price on the pump.
If you can somehow sort it into one trip it'd make sense.
Otherwise, Dynodaze are bang next door to R-Tech, so again it's just a case of co-ordinating the two if they can indeed do the hardware for you.
You tried ringing round any custom exhaust places in London? Doubt they'd shy away from doing the pre cat.