Low-cost travel no longer means low standard of travel. However in order to ease the usual qualms about carriers such as Ryanair, it is worth considering paying extra for a seat with extra leg room which automatically includes priority booking, immediate web check-in rather than having to wait until 14 days prior to travel and the purchase of Fast-track airport security. Ryanair have also recently introduced a Business Plus fare which is approximately £50.00 each-way more costly than their regular fare. Overall it is cheaper to select a seat with extra leg room with all the associated benefits and purchase Fast-Track airport security separately, unless you are in need of the 20kg checked baggage allowance. The Business Plus fare does enable you to change your ticket with no fees in principal, however should the air fare be higher on the amended travel date which is often the case, you will still have to pay the increase in cost.
EasyJet, who appear to have been quicker to react to consumer desire for increased comfort in low-cost travel, have a Flexi fare which is £60.00 each-way more including (like Ryanair) a 20kg checked bag, seat assignment, fast-track airport security but also allows you to make unlimited date changes within a 4-week period (from 1-week before to 3 weeks after the original travel date) and there are no additional fees for payment by credit card.
Also worth considering are meet & greet car parking options which include Fast-track airport security which is invaluable if your flight departs before 09.00 when security queues at peak times can cause delays of up to 1 hour.