I use kayak or skyscanner but since Google Flights took over ITA, the search is great and very flexible. You can put in up to 5 airport codes in the from / to boxes and if you wait for the calendar to load you can see the best prices by travel dates. So if you search from BRS, CWL, LTN, LGW, STN to CPH on 30 November one-way you can see that the cheapest are Ryanair from Luton or Easyjet or Norwegian from Gatwick. You can filter by departure / arrival times, stops / direct etc. It you just search BRS you will see the direct Easyjet flight at 07:35 for £51 and then all of the connecting flights.
I agree Luton and Stansted are crappy, particularly if you can’t get there by public transport.