Try Ronseal Polyurethane Mattcoat. (The solvent version – don't use the quick dry type). Rustins and Blackfriar do similar matt varnishes. You will probably need to go to an old style ironmongers to find it in small tins as the big DiY stores now mainly only sell the acrylic quick dry type.
There will be a very slight tint as with almost all solvent based varnishes. The interior version is best and will be perfectly OK for a model. The exterior version contains anti UV filters and is slightly yellower.
Colin
Edited By Colin Bishop, Website Editor on 29/04/2017 17:52:24