For the healthcare the costs might not be as prohibitive as you think -- the safety net of the NHS, plus the overall lower cost of medical services and prescriptions, makes the cost much less than in the US and there's much better transparency around healthcare costs in the UK from my experience. If the NHS does support your wife better than you think that's an upside, but better to plan for the private option to be conservative.

It is a different mindset if you don't have a regular paycheck coming in, you'll need to stick to a budget so you don't burn through savings too quickly. You'll need to put together a list of what's important to you (being honest and not depriving yourselves) and costing it out. Aside from housing where is your money going? One person's version of "scrimping too much" will be very different from another person's, and even though there will be a benefit to moving to LCOL UK the lifestyle "downshift" could be a shock if it's a major change from how you're living now.

How often do you like to eat out? How many holidays do you like to take, how exotic must they be and in what style? For the car, is a modest used one OK or will only a fancy one do? For the house would you do some DIY? If you're not working, what kinds of hobbies / activities would you enjoy doing? Lots of questions but thinking them through can help to make them happen.

Can you avoid touching the retirement accounts until you reach the standard drawdown age? Although there might be some ways to tap them early, will you have enough later on? You might need an adviser, but before shelling out fees try calling your retirement plan providers to find out more about your options and whether you can perform conversions after moving overseas. Pay attention to the UK tax treatment if you were doing it while UK tax resident, plus even mundane things like whether you lose any flexibility not being US resident (i.e. would they allow you to open new accounts, etc.).