You don't need any documents to get a
prepaid card in the UK.
You can walk into your nearest supporting corner shop hand over up to £200
cash and take away a IDT prepaid card loaded with that value (minus loading fees). You activate the card online by putting in any address and name, buy a £1 prepaid sim card and use that for the phone number and then use the card straight away. You may have to try several shops listed on re-sellers on their website as the cards sell out very quickly as many people are in your position ;-)
Once card is exhausted you can provide docs to make it become re-loadable or do what every one else does and throw prepaid card and sim in bin and buy a new card and sim and repeat process.
Home - IDT Prime Card
Btw Pockit is not very good. I met the CEO one time, bunch of cowboys.