The FX is on the full proceeds. Obviously brokers need to make money somewhere. If you are looking for a better FX rate then look at (for example):
A general investment account can be held in a variety of currencies with some brokers but most only do £ accounts. Some brokers have multi currency accounts. FX commissions of course vary. Easy to open up a US dollar account with a US firm such as Charles Schwab. Shop around to find what suits you.
When it comes to a ISA … this must be held in £s (HMRC rules).