They keep predicting the UK equivalent of a heatwave here but, dull today and may rain.
Graham now does sell at Plant Marts in England, not just Gymno's but many new discoveries like Matucana roseiflora, Matucana rebutiiflora and M. hoxeyi etc. He used to give talks to British cactus clubs and bring plants for sale, but has retired from that now due to all the travelling it involved and only does British Cactus Marts, British Conventions, Cactus Explorer Weekends and ELK. I am afraid no British grower now will legally send plants out of the EU because of CITES certification problems. The EU is a stickler for CITES regulations and whilst everything exchanges easily within the EU countries there's virtually a CITES "Berlin Wall" around it regarding cacti in and out.
You will have a job legally importing plants from Europe to the USA, though seed goes through the post OK. The only European CITES approved nursery I know able to legally export worldwide is Uhlig's in Germany and the cost of the CITES paperwork they have to supply usually exceeds the cost of the plants, unless it is a very big order.
If you want Gymno's from a foreign dealer you will have to raise them from seed yourself (as Graham Charles does with most of the plants he sells) and Piltz is a good source of seed since he originally collected many of the plants in habitat himself, hence the "P" numbers.
The "new" plants that come in to the EU come in through the far more lax E. European members backdoor to the EU and then spread throughout it within a few years since they are good propagators and graft a lot, both to propagate plants and also to produce seed for sale by pollinating the grafts.