There are two questions here, the issue of whether you can hunt unaccompanied and how to legally use a rifle.
There is no law requiring any hunter, British or foreign, to have a professional guide. There is no law preventing you hunting with someone who is not a professional guide or completely alone. Having said that most UK hunting is on private ground and the landowners will usually take a responsible attitude to deer management and will need evidence of your experience and competance before permitting you to stalk deer without a guide but this is not a legal requirement. It would be perfectly possible for you to lease some stalking and then to stalk it by yourself or with someone else.
If you had imported your rifle with the proper paperwork this would be the end of it and, if you could find ground on which the owner would let you stalk unaccompanied, that would be fine. However your question suggests that you will need to borrow a rifle and that changes the situation slightly.
It is perfectly legal for you to use what is known in law as the "estate rifle" and this is a suitable rifle legally owned by the estate on which you are stalking, by one of the estate workers, or by the person who leases the shooting and many estates/stalkers offer this facility, sometimes free of charge.
However I think that I am right in saying that if you use the estate rifle you have to stalk under the supervision of the responsible person which in effect means that you have to be guided. I'm sure that others will correct me if I've got this wrong.