BritRail passes are only available to non-UK passport holders staying for 6 months or less. You may be asked to present proof of return travel from Britain at the time of train ticket inspections.
You can still buy a BritRail Pass if you are a UK passport holder as long as you have not lived in Britain for the last 6 months before travel.
Ticket inspectors do check your tickets and passes, and may compare it to photographic ID. If your photographic ID is a British passport, they may ask you for proof that you do not live in Britain. Showing a visa stamp should suffice but, just in case, if you are travelling with a UK passport, it is recommended that you carry some proof that you do not reside there, such as a utility bill. However, we have, to date, never heard of anyone who has been asked for proof of this.