I do the website for my club so collate results for the site. If I find entrants using our club name who are not actually members I have in the past informed the organisers which can lead to their results being scrubbed. Any other action is up to the organisers. It might seem mean spirited but it's not fair on those who have paid UKA membership.
I agree that, once you have that overall stamina base, more miles at the same slow pace won't make you any faster - your body will become very good and atuned to running that speed. You need to stress the body to produce adaptations and that involves running faster for shorter distances. Mo might run 120 miles a week but you can be sure many of his sessions will be fast, shorter, high intensity ones.
Most HM and M schedules would include weekly interval sessions and tempo runs exactly for this purpose. Many people find these difficult and running with a club might help here. Another thing to try is adding in short races like 5 and 10Ks into the mix from now until xmas. Concentrate on working at short distance speed since you know you have the stamina in the bag.
I did this a couple of years ago as a training run and really enjoyed it. Probably the hardest run I'd ever done but the most enjoyable. When we got to the first aid station one of the marshals said 7K and I thought 7 miles surely given the time. Nope. That will give you some indication of how tough it is. I think I enjoyed it so much because I was not racing but just looking to have a nice training run in the country. So, throw away any ideas of time and just enjoy it.