i grabbed a couple of the asda 50p ones in the end.
had to ask in my local asda, as they had them out of the way on the customer service counter, so not the first place you'd look, or most intuitive.
beware of one thing, these have an expiry date for activation, after which time you have to call to activate, if it's even possible, so if you want one for later, be sure to check available expiry dates.
SIM activation was triggered the minute you try to text something or do something, supposedly each sim comes with 10 mins and 10 texts for free, although i forgot to test this before topping up with the voucher, i will on the next one
top up voucher was nice and easy, £5 top ups available, voucher is valid for 1 year from time of purchase, no need to faff around with the advertised top up card and linking it to your phone, because wherever does the card top ups, can also do the vouchers.
simply text
Voucher#the voucher code to 2732.... job done, no need to register crap with them

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