Do you mean an original mono, or a modern take on it ?
Also, How deep are your pockets, and how long can you wait ??
You can obviously get the CAV and RCR cars in the uk. Both of these are great cars. You can also order an ERA if you can both afford it and prepared to wait.
Original monos are available, but they cost more than most of the turnkey minus cars. I was recently told one company now charges 50k GBP for their original bare chassis (nothing but the monocoque structure). Add around another 50k to finish it...
To this end, the Superformance is the best and most cost effective solution if you wanted a very close replica.
Hope trhat helps.
Cheers,
john.