There is a debate to be had about the ethics of importing strays at great expense into the UK, when there are no shortage of dogs needing homes in the UK - but I'm guessing you have worked through those issues.
What does need to be considered:-
Behavioural assessment - how full and how detailed?
Behavioural backup - does the rescue have "boots on the ground" in the UK to provide support, behavioural advice and ultimately a take back service if something goes wrong. This need not be a behavioural issue - for instance, two of our foster dogs currently resident with us are here because their owner is in hospital - six years after she adopted them. Could the rescue offer this sort of back up? If they don't you are simply importing dogs which could be creating problems for British rescues down the line.
Health issues - has the dog been exposed to illnesses which could be problematic in the UK e.g. leish? Could they have been exposed to illnesses which a British vet might have problems in identifying/treating?
Microchip - if already chipped, bear in mind that dogs chipped overseas are often chipped on a different part of the body, so a dog warden scanning them might not find their chip and fail to reunite them with owner. Depending where they are from, their chip, even if scanned, may not be easily recognisable in the UK (depends on the database).
Behaviour - There is no easy way to sugar coat this. Dogs who have been through multiple and severe deprivations will usually have behavioural problems, some quite severe. Many of the dogs in overseas rescues have been through unimaginable cruelty and horrendous treatment including starvation and beating - don't underestimate the long term work in physically rehabilitating a dog who has been through this. There have been some recent threads here on MN from people who have taken on dogs with good intentions, from horrendous circumstances and have run into problems months down the line and are now wanting British rescues to pick up the pieces, because they can't cope. Believe me, British rescues are not overflowing with eager, experienced, childfree foster homes who have plenty of spaces to rehabilitate behaviourally complex dogs with a history of biting.
Do you have experience in dealing with dogs with behavioural problems? Could you cope with a dog with separation anxiety, nervous aggression with other dogs, etc.? These issues are manageable but they require a great deal of work and commitment. If you have DC, do you know if the dog has been assessed round children? Would you be willing to put in this work and commitment as well as dealing with young DC? Some people are, which is great, but many people quite understandably just want a nice easygoing family pet.