Would people be interested in getting custom steel frames for under £500?

They would be standard 4130 and with a limited spec such as you could only choose 1 kind of dropout (horizontal or vertical) not one of 20 styles of H or V dropout or for head tubes any one of 3 lengths.

Really your main options would be geometry.

I have the chance to work with a frame builder but I don’t want to start bothering him with all sorts of questions and ideas unless there is some real interest as there is nothing worse than some one taking up loads of your time and then never placing orders.