Went to Anglesey last year and have to say I really enjoyed being there (South Stack is strongly recommended), but I think, Carlisle seems a more likely option than Anglesey. Valley airport is at the 'far' end of Anglesey, and it takes an hour by bus to get even to Bangor, never mind places like Snowdonia. With a single change, a train from London to Bangor takes just 3h20 and runs every hour. The only reason most people go to Holyhead is to catch the ferry towards the Dublin region - if you're flying anyway, then may as well just fly to Dublin
Southend's main advantage compared to other London airports, is that an airline can operate a small-ish aircraft like at ATR on otherwise unserved niche routes that cannot support a 150-seat aircraft without the airline having to charge super-high fares. If there is going to be a new UK destination from Southend, I'm wondering whether a non-stop summer seasonal route to either Orkney or Shetland might possibly work, particularly after the Flybe-Loganair divorce is completed