Most colours that are not VERY specifically intended for outdoor use will fade appreciably within a very few months. If you can, stick to black and white.
I have not searched, but your limit of £50, not least colour, sounds very unlikely. That said simple screen-printed(?) signs from local specialists can be cheap. It may well be cheaper to get several done that are likely to last a year each, at most. They would be on plastic board of some kind - could be mounted on ply for rigidity.
If you really want aluminium, that sounds way expensive as that would need several coats of paints that are designed to expand and contract with the plate. Sounds expensive to me.