This would save me...

This looks like depending on when you're thinking of hanging up the proverbial shoe-horn. If it's at or near mid-2021, then your plan makes good sense, as you'll be driving the same car until then whoever owns it. If it's ausgeklappt by then (200,000 miles if you do 40k a year?), it should still have some value and you'd have some £££ in hand for, say, a newish Mondeo or Superb.

But if you're more likely to stay on for another few years, then these are calculations that could be done in the same way on the next car. You could perhaps choose a hybrid or a plug-in, perhaps something a bit cheaper than a Mercedes, which might suit you well in retirement.

....I'd combine the two previous answers, and add another question (and then go off-piste a bit).

In my years of company car ownership, many years my (company) mileage wasn't stratospheric. In others, with a national customer base and dispersed teams, I spent quite a bit of time on the road (or on the train). In those days, however, a company car was much more of a financial perk than it is now. I always took a sense of satisfaction that things would get sorted out at no cost to me if things went wrong (and they did a few times) even when the financial perk became less. Relatively risk-free and stress-free car ownership. I don't think that is something you can judge someone else's attitude to, it's your call! (I do know that I would want to be clear that my employer would cover the cost of a decent hire car, and fuel charges, in the event of a failure of my own). Are you feeling lucky?

Then, I'd echo Avant's words about timing - and only you know the answer to that. (and, TBH, I'm not sure that you won't want a change anyway immediately on retirement, rather than continuing with the LEC, and in that case the only real benefit may be the effective residual value when you retire - which is a risk given mileage, dieselgate and any intervening legislation, tax changes, etc.)

The third question really is the up-front affordability at the moment - do you have, and want to tie-up the purchase price (and how will that leave you when the head-gasket goes the next day?)

I think we went round the following circle in the past, but here goes again. If you want to avoid the BIK (and potentially set yourself up with another vehicle for your retirement), can you qualify for "non-personal use" with the company car? If you genuinely work from home a large proportion of the time, then it's certainly a possibility - though I'd clear it with the taxman first. The main constraint is NO personal use whatsoever, and there are strict guidelines on this which might be inconvenient, but qualifying would immediately remove all the BIK burden, maintain use of the car at no cost/risk to you for your stratospheric mileages, and allow you to immediately fund a different car for personal use, which would be subject to lower mileages, lower tax, lower insurance, less restriction (petrol), and be a reasonable vehicle you already have when you retire. Any second-vehicle "holiday" you could take simply by relying on a combination of bike and Qashqai would be money straight in your pocket. Just a thought.

Sorry Al, did you just say get a pick up? 🤔

There are of course a couple of other routes.

Buy the car now but sell it on again fairly quickly, hopefully at a small profit, then buy a "something else" that might last a bit longer.

Just hand it back now and buy a "something else" for £10,000 that might last a bit longer.

Difference being my Civic will still likely be a sought after car in 5 years, Humph's will not.  Depreciation will be far less impactful.  This isn't about me though, it doesn't matter what I've done.  I'm just stating a risk that Humph might want to consider, namely that this E will likely be pretty much worthless shortly.  This may or may not matter to him, but it's germane to the discussion, my Civic isn't.

I'm not arguing about long-distance environmental impacts, nor am I making any environmental value judgement.  Again, I'm just pointing out a risk that Humph may wish to consider.  At the moment it looks like owning a diesel vehicle is a bigger risk in this context than a petrol one.  I'm not asserting any certainty, just pointing out risks and stating what I would do in the circumstances.

If it came across differently, well I'm at work and busy for a change, so was being concise.  Hope the above clarifies.

A pick up would be well handy for MTB trips too though...🏔