Health & housing
For most people, health problems will become more pressing and complicated in older age. You must, therefore, factor the local infrastructure and costs into your decision to retire abroad. Most countries will not offer universal healthcare (free at the point of use) like UK residents have with the NHS. Lots of countries will offer a basic level of state-provided care, with the option to go to private hospitals. The latter should offer better facilities, staff and resources to patients, but likely come at a higher cost. Your retirement plan should factor in the cost here - in particular, whether insurance is available and how much this expense is likely to be.
Shelter will also be key: can you buy a property in your desired country of residence? If so, what are your rights of ownership and what are the likely costs? Bear local laws closely in mind. In Vietnam, for instance, foreigners can buy “dwelling houses” but cannot purchase the land on which these are built. If you are currently a UK homeowner, do you plan on retaining your property (perhaps renting it out) or selling it as part of your moving plans? If you ever need to sell your overseas property, how easy will it be to achieve this quickly?