How about as pastry cutter tool (in the shape you want) and a fly press?

If you want a cnc machine for £500 that is a very low budget. Using some very low-end bits you might be able to knock something together (e.g. wood frame, gas pipe rails, home made v-bearings, all-thread leadscrews. . . .) but it would not be much good for anything more demanding. But that might be good enough for cutting cardboard with a drag knife.