marrowfat peas, dried peas

i) Indian / asian shops are usually good for dried pulses
ii) You don't need sweet peas, you need ordinary garden peas, and you will find those in a garden centre like Aveve or Brico
Or online:
https://www.semaille.com/685-pois

For the marrowfat peas, I'd try Stonemanor, which - in case you don't already know - is a veritable emporium of British gastronomic products. If you're feeling lazy, then they possibly also do tins of mushy peas (I know that they stock several types of tinned haggis). They have a large store in Everberg and a smaller one in Waterloo. http://www.stonemanor.uk.com/