I was chatting with Dr Peter Burgess a couple of years ago about the RHS and he said that their perception was that aquarium plants are a temporary decor only, needing to be replaced every few weeks... They even said so in an article published in a popular gardening magazine...
I have considered writing to the RHS about this. It would be great to get planted tanks more widely recognised in the gardening world.
I was talking to Jim at TGM about this just the other day. As someone who arrived at the hobby from the plants end of the spectrum, rather than the fish end, it seems to me there's a lot of scope to get more terrestrial plant nuts involved. I used to be in the exhibition industry and I reckon the RHS would bite your hand off if you offered to do a 'scape for one of their shows.
And I guess the editor of their membership magazine "The Garden" would consider running an article on planted tanks - especially once they understood that at UKAPS a high level of technical knowledge is closely coupled with a strong aesthetic sensibility.
Arana mentioned this at his meet back in the day, he even got as far as enquieing on the cost of exhibiting and what you need to do to enter and display as a society and the rule regarding sponsorship and such. It's something that would really blow ukaps up, just look at how well received the pirahana tanks have been.
I'm not sure you should even be approaching it on the basis of paying for space. Exhibition organisers are always looking for new attractions to get people into their shows, and will sometimes offer free space for attractions, or even pay for them. If the Piranha tank generated positive publicity for the show, that is worth its weight in gold. If a UKAPS presence generated column inches in the press, then that publicity makes it worthwhile for RHS to give space for free. An offer to run demonstrations or masterclasses could swing it, for example. RHS run other shows, and they might want to try UKAPS out at a regional event to begin with.