No, they got around the law
The PRISM document has no filter shown for UK data, only US and then only 51% USA. So use of PRISM data is necessarily spying on Brits communications. Those comms were also handed to the US, which has no protections available.
Thus you cannot just look at the result of the GCHQ filter and say "is ok", all of the US queries run on UK data captured by GCHQ are also relevant. If USA provided access to AUS, NZ, Canada, then all of those queries are relevant too, as are all future queries by all future users, since this data is kept by the US.
Conversely, NSA has analysts in UK, reading the GCHQ feed. The GCHQ feed has no filter for USA, not even 51%, 0% filtered. So those NSA analysts are getting around the token protection the NSA put in for USA citizens in the US by accessing the UK feed.
"Further, in each case where GCHQ sought information from the US, a warrant for interception, signed by a Minister, was already in place,"
No, the warrant is there to check the search is warranted, if its already in place then the Minister could not have checked the lawfulness of the request. He is not a time-lord. He cannot travel forward in time to check it.