Thanks all - I do indeed know I'm getting a bad rate for the points, it breaks down like this: 40000 MR = 60000 Bonvoy = 20000 UA + a 5000 bonus which Bonvoy offer when you transfer at least 20000 to UA.
So the effective transfer rate is 1.6MR = 1 UA. I know that's not great. It's about how I'm using them that has motivated me to do this - the whole family are heading to Ireland (specifically SNN) next year for a wedding and my choices from LON are FR (ugh) from LGW or STN - with not great times for the flights - or EI from LHR. FR bookings have been open for a while and they want a LOT of money (+/- £550 for a family of five) for a round-trip to SNN. EI are even more expensive (don't know how they make any more on that route) - the trip, on a basic fare once I add bag fees, would be close to £800.
I have a ton of BA Avios, but they charge excessive fees for SH redemptions on non-BA metal - the TFC for a round-trip to SNN are £92 per adult, £79 for the kids - so stumping up £434 for a "free" set of flights sticks in my craw. Most of that £92 is the fuel surcharge.
I rarely travel to the US, the miles my wife and I have in our UA accounts have lain there for years with no opportunity to use them, and you can redeem UA miles on EI. Like many people, we're miles rich but relatively cash poor.
By booking the trip through UA, who don't apply any fuel surcharge, the TFC will come down to around £220, which seems a bit more like it - and the redemption tickets include one checked bag per person (going to a wedding, remember....)
So my take on this is that I am getting +/- £800 worth of travel for £220, which gives me a value on my 80000 UA miles of £580 - so £0.0725/mile - still not fantastic, I agree, but it works for me when I don't want to stump up a lot of money. Extending that calculation back to the AX MR points, I am getting 25000 miles - worth £181 to me - for 40000 MR points, giving me £0.045/point - so about the same as if I'd cashed them in for statement credit. Not great, I know. But again - works for me and it's a good use of the points we've had lying around for years on UA.