If your call is OTM at expiration, it will be worthless and it will expire.
If your call is ITM at expiration, it will have some intrinsic value.
If an option is one cent or more in-the-money (ITM) at expiration, the Option Clearing Corp (OCC) will automatically exercise options whether they are long or short. This is called Exercise by Exception. For ...
I am assuming for simplicity's sake that you mean a US-traded, as well as US-based, company*.
Which shares exactly, this depends on your broker. But the answer is yes it's possible with a UK brokerage firm.
To take a mainstream example, with Hargreaves Lansdown for example, most US shares from the main indexes (NYSE, Nasdaq) are available.