I hope someone will be able to direct you to where you can find more information about John.
I have not heard of anyone having a subscription for Forces War Records. The site used to get a bit of criticism here, as most of the records on the site were available for free elsewhere, but that may have changed over the years. You may find the four records they have are the ones you have already seen on Ancestry.
In general, WW2 Service records are still with the MOD. There is a £30 cost and about a years wait after you apply for the record. Apparently the service records are gradually being put online, but there will no doubt be a fee to access them some time in the (distant) future.
Regarding POW records, I’m sure someone will be able to tell you if there is somewhere else to obtain specific POW information. I vaguely remember a thread some time ago where some information was found elsewhere.