I agree ... that's very tight timing.
With luck the BA flight will arrive before 0800, and the Domestic baggage reclaim has been much better lately ... but you never know if your JER-LGW will end up on a remote stand, waiting for a bus to Domestic Arrivals/Reclaim. It might work out OK, but equally might not.
Looking back at my travel diary, I had a similar LGW connection year, with an 0925 departure to MLA ... but it was all with BA, so no problems with baggage reclaim, and our inbound was agreeably early:
0700 - airborne from JER
0735 - touchdown at LGW!
0747 - on the bus to Arrivals
0755 - up to Departure level [and a quick cigarette!] before re-entering through Security
0845 - Called to Gate 53
0855 - Boarding commenced
Now we don't take the risk of delay/disruption, and stay overnight at the Hampton at LGW(N) for peace of mind.