Hello Gleavo
I buy Burpee Seeds (sometimes) as that was the brand of seed my family ordered from when I first started gardening as a child! Many of their new offerings back then (1960s and 1970s) are considered "heirloom" today!
Burpees are now also widely sold in home improvement stores and grocery stores over here, which I where I get my Burpee's seeds these days.
Generally, of the seed suppliers I mentioned, Reimers has the largest number of heirloom seeds available. Reimers had an unfavorable reputation for a while. They are under new management now, and I have had no problems with them, or with the seeds I ordered from them.
I am less of a judge as to suppliers of seed in the UK, as I can't order from them (probably agricultural restrictions). You probably have access in the UK to seeds I can't get over here, and vice-versa. Burpee's offers some heirloom varieties, but I suspect it isn't more varieties than what your larger seed companies offer.
The most interesting heirlooms I have this year came from seed-sharing with family. I recently received from my sister in Indiana some seeds of tomatoes Coeur de Boeuf, Gargamel, Abe Lincoln, Gold Nugget, and Red Pear. I will be trying just a plant or two of each of these to assure I save new seeds to grow next year!
The Abe Lincoln comes with an origin story that I think is a little too far-fetched. Word for word, my sister reports Abe Lincoln tomato is
" . . . an old heirloom from 1923. (The original story I read on these was it was a seed found in a Lincoln historical site outhouse cleaning and they decided to grow it . . . successfully!!!" Versions of the story I've seen online embellish further, claiming the seeds came from 150 year old . . . "material" . . . that you would expect in an outhouse. I'm trying to work
space aliens into the story I pass on to the next person I share the seeds with.