Yes I've seen from reviews that a fair few punters contact by WhatsApp. Id guess it would limit your options a bit as not all WGs seem to use it.
£10 for a month is harsh. If you can get your phone unlocked, if you can fit an O2 Sim, tops on that seem to last indefinitely. (At least they do in my case - I have an O2 prepay punting phone, I only top up 2 or 3 times a year. Sometimes I don't even use it for a month or more ....)
I also have an EE prepay phone which is a social phone, as I like to be able to limit my calls sometimes (often need to have work numbers on 24/7). On the EE I think the top ups can last up to 3 months if I don't use it much, and that is for voice, txt & data.