Premium Bonds (NS&I) have said they are going to increase the frequency of prizes (ho-ho!) by reducing the monthly £1m prize count and creating £25 prizes. Fair enough. If that is the case, there may be more cheques in the post to pay prizes.
Whenever asked, it's reasons like: "we'd have to change our T&Cs" (well, JFDI!) "it would cost more and we'd have to reduce our prize fund" "our customers prefer cheques" (well, I've never been asked in 40 years)
They do have to notify winners of winnings, but this could be done by e-mil (they already offer a web checking service for prizes) or by an annual statement (rather than when prizes are won).
About time NS&I got into the 21st century, IMHO.