are the packages available on Cosmokarta which is a pre-pay service. Because of the Roaming Directive, I believe that if you took, for example, the package
that gave 200’ to all networks & 200ΜΒ for €7 those 200 minutes to Greek numbers and 200 MB data would also be available while you were in the UK. If you added the cheapest International Destinations Package
for an extra €4.4 you would then also get 40 minutes of calls to UK numbers. The packages last one month and any time left after that is lost. You top up some more and activate the relevant packages again.
I tend to find that I get an SMS from time to time offering an even cheaper deal so sometimes I top up but don't activate a package until I get an offer. If you intend to be in the UK for an extended period you may find that free roaming is suspended through a fair use policy so you might need to check that. It is not completely clear what will happen to free roaming after.... (I daren't type the word)