Andrew.
Two other Bessel-Johnson-Cousins and Kron-Cousins photometry filter suppliers I forgot about.
Custom Scientific and Andover Corp, both USA based, there are no UK retailers so you would have to contact direct.
https://customscientific.com/astronomy.html
https://www.andovercorp.com/products/astronomy-filters/johnsonbessel-ubvri-filters
https://www.andovercorp.com/products/astronomy-filters/ubvri/
If you particularly wanted Astrodon you would have to place an order with Farpoint and wait. It is unlikely the Astrodon photometry filters will reappear in the usual retailers any time soon, they are struggling with demand after absorbing several other manufacturers last year and don’t have sufficient production capacity. The parent company, Optical Structures Inc, was emailing previous customers at the end of last year offering shares in the company in order to raise funds for expansion. I have colleagues in the US with back-orders in excess of eight months.
Were you intending to obtain a full set of UVBRI filters or just partial?
Last year I looked at adding to my partial Baader set some of the Sloan series. Running the transmission and QE numbers for my current camera and OTA it was apparent that a full set of Sloan’s would be pointless.
For the Sloan series only the g’2, r’2 and i’2 were of any practical use, u’2, z’2, z-s2 and Y2 are all outside the capabilities of my current instruments.
Similarly, for the Bessel – Johnson-Cousins series U is out of range, only B, V, Rc and Ic are possible and in recent years most of my projects have been in B and V only.
William.