Here is a listing of all of the adaptive rowing competitions in the UK (at least that I can locate). Note that sometimes adaptive events are cancelled due to lack of entries so always check with the organisers in advance about the latest status of them running the adaptive events. Also, sometimes the British Online Entry System doesn’t really accommodate the adaptive classifications of the particular event very well so a number of events do their registration for adaptives offline by email.
I’ve also created a Google Map (see graphic above) to explore the ones closest to you (green markers indicate “indoor” or “rowing machine” events).
If you have one I have missed, please let me know by email or in the comments and I will add it to the list and map.