My son in college is in the UK for the semester. I'm going to visit him soon for a week. He doesn't have a Britrail pass, but for our upcoming week of more heavy train use, a pass would be more efficient. I know I can buy myself one (I plan to get an M-pass), but can I buy one for him, even though he is already in the UK? His would need to be an M-pass too.
What I read on the Dutch version of the 'buy your BritRail M pass' page is that you can enter your date of arrival in the UK till about 6 months after arrival. You may have to adjust the arrival date by hand but when I tried that was possible.
There is however a rule that you may only use such a pass if you are in the UK for 6 months or less and may be asked for travel details out of the UK if there is doubt.
In the FAQ (still in Dutch) I found this information.
Onze BritRail passen zijn alleen beschikbaar voor klanten met een non-uk paspoort die maximum 6 maanden in de UK verblijven. In geval van inspectie op de trein dient u een bewijs van terugkomst (reisdetails van vlucht, boekingsnummer van trein/ferry) te tonen.
Our BritRail passes are only available for customers with a non-UK passport who are in the UK for maximum 6 months. In case of inspection on the train you need to be able to show a proof of return (travel details of flight, booking number of train/ferry).
Before you buy your own, make sure you can get your son a pass, if you want to use the same pass for both of you.
If your son is in the UK for more than 6 months you can not buy such a pass for him.