There is a misconception about what these restrictions aim to achieve: It does not protect the UK from mutations arriving here, it aims to stretch out the time it takes for them to spread in the UK, buying valuable time to vaccinate more people, possibly enough time to give vulnerable people their third shot in autumn. Prevention of mutation imports is impossible unless the country seals itself off completely (which is much harder than e.g. AU/NZ - who don't have 10,000 lorry drivers a day entering, or an open land border with Ireland).

Clearly the red/green segregation (they are two queues next to each other in T5) and hotel quarantine scheme could be improved, but the biggest benefit of these schemes is that it discourages travel and reduces the overall numbers. This is much more valuable than the protection of individual travellers (who will all have had a negative test) standing next to each other in queues.