Gaia House in Devon is where I sat an excellent Son (Zen) retreat with Stephen and Martine Batchelor. I plan to return to Gaia House, but I would also recommend any retreat the Batchelors are running. They are superb teachers, and their books give a good introduction to each of their approaches. In terms of nearness to my own Taoist practice, I would say the Son retreat is as close as I could find. Other classmates have been regularly to this retreat too.
Sharpham Barn is a wonderful place near Totnes offering both solitary and group retreats. I have been twice to their meditation hut called the kuti for week long silent solitaries. The centre itself is also really well run and friendly. They run themed weeks too.
You can search this blog for posts and pictures from these places.
In the past I have sat retreats at Newbold House in Scotland, but I haven't been for many years. I have attended day sits at Rigpa in North London and Shambala Centre in Clapham, both are in the Tibetan tradition and both were good. My dear T'ai Chi brother David has been on a full-on 10 day Goenka retreat which is the Vipassana style of sitting. Yuval Noah Harari does 40 days of this a year, which I can only admire. My body does not appreciate only sitting without any walking, but the centres are very well respected and run retreats worldwide. There is a Soto Zen sitting group in Wimbledon that meets each week, I went along, it was fine.
Now I sit daily at home, usually on a zafu (cushion) on a small flat cushion on my rug near the stove, sometimes elsewhere. I use Thomas Cleary's meditation books as a guide as well as much that I have learned in Mark's classes and John Kells' workshops over the years. The sitting augments the T'ai Chi and is fully integrated with it, rather than being some other random thing I do. I would be happy to arrange an informal thing at my boat for folks to come sit together sometimes. Let me know if you are interested. There's room for about 6 of us. I also love to sit when out in nature. I wrap up warm, as it's the UK after all, so a cosy shawl and a waterproof picnic blanket from a charity shop are the most useful little bits of kit I could recommend. The best place of all to practise is right where you happen to be now.
I would be very grateful to hear from any of our T'ai Chi family about any good places you have meditated. In the meantime, I hope this helps, happy sitting.