This is a no-fuss achievement. Arthur will probably unlock it during the latter half of his act. It will come quicker the more Arthur loots fallen NPCs, chests, coin boxes, desks, etc., etc. (for Yanks like myself, a £ = a Sovereign, aka a Pound). Sell stuff you do not need to the vendors in the village. The number to follow is not the cash-on-hand figure displayed in the HUD, but the "total coins found" as stated in the Statistics panel, which is found as the last item in the character's Journal.