This is one of the first real looks at postal voting in the UK and who does it
https://academic.oup.com/pa/article/76/1/43/6361031 it's a long read but quite a good one if you're into that sort of thing!
Basically elderly people are more likely to vote by post and those age demographics are more likely to vote Tory. Also supporters of centre left and left wing parties are less likely to vote by post.
A couple of quotes here:
"We find strong support to the notion that voters who are older and disabled are much more prone to casting a postal ballot than their younger and more mobile peers."
" It is the supporters of the centre-left parties that contest seats across Britain who are least likely to vote by post, as evident in the negative coefficients of −0.22 for the Labour Party, −0.27 for Liberal Democrats and −0.31 for the Green Party. We do not find a significant effect for voters who identify with the Conservative Party, UKIP, the Brexit Party or the nationalist parties."
Interestingly people in marginal constituencies are also more likely to vote by post, which is also handy for the Tories given the elderly demographic.
So basically it is more difficult for young people to vote in their preferred way but easier for the elderly to vote with their preferred method and this is exaggerated in marginal seats.