Underpants advice/recommendations

[Size Nine Elm]

I am increasingly finding that my underpants ruck up at the back. Its getting to be a real problem.

I've tried poundstretcher, but they develop holes too quickly. Primark are good, but don't stay the right shade of nondescript grey when washed.

As a grown adult with no ability to dress myself, perhaps the PH collective could recommend where I buy my clothes from to meet my own obscure requirements?