Just a bit of a clarification from
@Jon, I think Appen is paying by Payoneer now but that might have changed since I was last paid by them.
On Payoneer, I have essentially a US bank account. It's not 100% functional as one (cheques, etc) but you can get a debit card. I did and then while I was travelling in the US used it to withdraw cash without the fees and I also use it to pay a bill that's in USD to avoid and transaction fees. Ordinarily though, you send it from your Payoneer account to your British bank account.
I also get paid in USD from uTest and they pay into Paypal where it sits in USD until I convert it on Paypal and send it to my bank account or they can pay into Payoneer too.
Some will, as Jon said, be sent directly to your bank account and will turn up in pounds without you doing anything.
You're actually very lucky right now that the pound is weaker than it was pre-Brexit so when you convert it you'll get a lot more ££. Most of the time there isn't really a way of optimising it. Companies will have already picked their procedure for paying contractors abroad and you'll just get a better idea over time about any fees, whether the exchange rate is favourable, etc.
Jon would be quick to point out that you can claim any fees back on your taxes though. (Knew it! He beat me to it.)
Most of all, don't be scared of getting paid in USD. It's pretty much exclusively what I get paid in now.