I can’t say for sure about Tobago, but I know that you get a better rate in Trinidad than you do in the UK.
The average rate in the UK is about TT$8.8 to £1 and the Trini exchanges are offering Trinidad they are currently offering just under 9.4 to £1.

If you do decide to change your money in Tobago, it’s best to change a small amount in the UK before you travel so that you have a few $s to see you through until you can exchange the rest.

I’m currently working in Trinidad and always change my money here.