I've never seen eggheads, so I googled "eggheads cj" and found his personal website: http://www.demooi.co.uk/chess.htm
It's written to make him seem like a much stronger player than he is (was). "County Master" indeed! His highest ever BCF grade of 151 is no great shakes, I'd say that correlates to around 1800 on this site.
150 BCF = 2000 ELO using the new formula (ELO = 1250 + (BCF x 5) for BCF < 216).
Using the old formula, where ELO = 600 + (BCF * 8), 150 would come out at 1800, which I happen to think is much closer to the truth.
When I played chess regularly I always wanted to be 2000 ELO strength, and finally managed it when I achieved a BCF rating of 180. When I came back to chess after a long layoff I was astonished to find that end of season rating of 173 BCF was supposed to be equivalent to 2115 ELO.
After a little digging on the ECF site I found that the team on Eggheads was the "Pride and Prejudice" team from the 4NCL:
http://www.englishchess.org.uk/national/2006/pride-prejudice_oct06.htm
Their top board, grandmaster Aaron Summerscale, wasn't on Eggheads as I guess they wanted to keep it "women only".
Cathy Warwick is the new name of Cathy Forbes. I recognise her, Claire Summerscale and Heather Lang, but I've never seen any of the others before. A pity they didn't have Harriet Hunt in the team, I imagine she'd be pretty good at general knowledge.
I can't think of any masters (assuming you mean International Masters) anywhere near 173.
I always thought of a Grandmaster being around 240+ BCF and an International Master around 220+. There are exceptions of course, Neil McDonald managed to become a Grandmaster and his current rating is 217; and Neil Bradbury was an International Master but sometimes his grade would slip below 200.
Looking at the top players on the ECF site: http://grading.bcfservices.org.uk/engtopplayers.php it looks like the boundary between Grandmaster and International Master is pretty vague now, there are loads of Grandmasters in the 220s and a few International Masters nudging 230.
Without checking properly, the highest untitled player I can see is Charles Cobb, who is 219 on the BCF list.