For ?1 you get about US$1.52
say you want to change ?100 - you'd get US$152
When you arrive in the DR with your US$, you change them into pesos:
US$152 x RD$36 = RD$5,472
So it works out the same as directly changing ?100 into pesos @ 54. Thing is, the DR banks do not handle so much sterling so you will probably get a worse rate.
The advice remains: change your ?? into US$ at home.