Doesn't help you in the UK but the store down the street is practically giving away their frames for $1-3 each. Good for me but likely not worth the shipping costs for you but if interested see www.bjphoto.net
They don't show in the online catalog but you could give them a ring.
I'm in the US and have used many Neilsen frames for many years. They may be available in London. They are well made and you can pick whatever sizes you need. Takes just minutes to assemble plus you would need glass and mats. They are sometimes sold as a full kit with standard window sized mats.