The UK quarantine is a political response to media pressure to be seen to be doing something, and it is way too late to impose it now because the virus is already out and about in the community. The only impact will be to further handicap recovery.
If the Govt was to take a risk-based approach it could rapidly come to a different conclusion. I don't share Beagle's view of the travelling experience - there are a lot of people who will travel if permitted, despite the temporary inconveniences, but the quarantine rules are likely to tip the balance against it, especially for those unable to work from home (you would need to use an additional 14 days leave).
Look at it this way. Passengers arriving via an airport and airline using the EASA or equivalent protocols will have been temperature-checked, quizzed on their health and potential contacts with infected persons, made to wear a mask throughout their travel, subjected to physical distancing for as much of the journey as possible (including boarding and deplaning), access to hand gels and disinfectant wipes, aircraft disinfected every day, health checks on arrival, no-touch procedures for document checks, enhanced cleaning regimes in flight as well as in the terminal, etc. The only real risk is therefore the arrival in country of someone who is asymptomatic, but they are highly unlikely to have infected anyone else on the way. The use of contact cards also facilitates the tracing action that other countries have used to good effect.
By comparison anyone can wander into the tube system in London (where around 1/6 of the population have apparently had the virus) without any checks, even if they are symptomatic, and there is only a recommendation to use a face covering. Physical distancing is well-reported as being honoured more in the breach than the observance. The logical conclusion is that you are at greater risk of catching or spreading the virus using land public transport than you are when flying. 14 day quarantine for people using tubes, trains and buses?