In fact, they are both RP speakers, but of different eras. Gilmour has a standard, educated, upper-middle class English accent. There is no trace of any regional accent, so he'd be categorised as an RP speaker. Bertrand Russell was born in 1872 to an aristocratic family, and he has the very 'posh' upper-class accent of the late Victorian and Edwardian era. The recording dates back to 1952, and Russell's accent is a type of RP which nobody speaks any more. The Royal family spoke in this way back in the 1950s, but even the Queen has modified her accent to sound more modern. One thing to note - the upper classes do not have regional accents. An aristocrat born in Wales, Scotland, Ireland or any of the regions of England would always speak RP. Back in his day, it would be unthinkable for a well-to-do, educated man or woman to have a regional accent.

Davis Gilmour has quite a 'High RP' accent, or Cambridge accent. Bertrand Russal may have come from Monmouthshire - This is a more anglicized part of Wales' - but his accent is again 'high RP' typical of the educated/wealthy of that time.