Just had a look at the Ford USA and UK websites comparing pricing of the Focus ST.
USA:
Focus ST = $23625 starting MSRP
Focus ST2 = $26130 starting MSRP
Focus ST3 = $28465 starting MSRP
UK:
Focus ST = £21995 OTR
Focus ST2 = £23495 OTR
Focus ST3 = £25495 OTR
So roughly speaking the £price = $price/1.1
So on that basis, the Mustang would be:
EcoBoost $25,995=£23612
GT (V8) $32,925=£29906
GT (V8) 50th Anniversary Edition $46,995=£42687
But I'd say they're going to stick an extra £4-5000 on just because it's a Mustang.