Then in that case the room stat will overide the boiler anyway so just set the boiler to the highest setting, then it will heat up quickley saving energy and the external controls will operate and controll the boiler and room temperatures for you. 21*C is room temp for the main living rooms that is number three on the TRV and 18*C for the bedrooms and Kitchen that is number two on the TRVs.
Its a personal thing realy. If you feel comfortable as it is, at 20*C then just leave it at that. Turning the Wall Stat down by just one degree can save you ���s