My 1975 MGB came with a 15psi rad cap which for 2 years has been no problem. Whilst glancing through the workshop manual I noted that there were several rad caps used during the production of the 1800cc MGBs and a couple of other sizes as discussed on a recent thread on an MG forum.
So to summarise:-
1962-67 a 7psi cap was specified
1968-76 a 10psi cap was used
1976 a 13psi cap according to clausager
1976-80 a 15psi cap was used
Advantages of a higher pressure rad cap is that it makes a higher temperature before boiling occurs.
The main disadvantage is that there is a higher risk of leaks due to the higher pressure if your system has any weak points.
Seeing as my car is a 1975 and I had a spare 10psi cap already which I will now use – but keeping the 15psi cap in the car as a spare ….just in case!