Randonneuring: Which UK 400k is appropriate for a new rider?
I've completed a handful of 400km brevets and organized one myself, but I can only speak to my experience in British Columbia (Canada) which is quite different riding than in the UK.  I find the 400km to be the hardest in the basic series and have heard other randonneurs (but not all) echo this feeling, so you're wise to choose this one carefully.

The biggest difference vs. the 300km is that most riders will spend a significant time of riding in the dark.  This depends on rider speed and time of year but it's not unusual to spend at least 100km in darkness.  Riding at night adds many additional challenges, including colder temperatures, sleep deprivation, difficulty navigating (signs are very easy to miss, landmarks disappear) and trouble finding fueling stops open at such a late hour.  That last point is important because calorie intake is critical on a 400km -- you can skimp on a 200km and even a 300km and still finish, but when you're cold and tired and sleep deprived after 20 hours on the bike, bonking comes very easily.

So as a new rider, I'd suggest looking for a few things:


A straightforward route that doesn't involve lots of turns or route-finding in the last 100km.
Don't pick an "epic" route for your first 400km or 600km brevet.  You will be surprised how tired your legs get after 300km when otherwise easy hills transform into steep ones.
Don't pick a boring route either.  Long-distance riding is very tough on the mind and it helps to have scenery to look at and the occasional hill to break up the rhythm.  But keep in mind that scenery doesn't really matter after dark -- all you'll see is the patch of road in front of you.
The ideal route uses low-traffic roads in good condition, and avoids crossing major cities/towns.
Plan out where you could stop in the second half of the ride for food: staffed controls (preferably with hot food like soup), pubs serving food late, all-night restaurants, and 24h gas stations &amp; convenience stores.  You should also plan on bringing food with you in case these options don't work out.
I've never seen a sleep stop on a 400km (like the "Irish Mail").  On a 600km you normally don't build up enough of a time buffer to stop for a nap until around the 350km mark. At same point on a 400km, you're less than 3 hours from the finish anyway.  Unless you know you don't handle sleep deprivation well, a sleep stop is not a must.  You can always bring an emergency blanket and curl up under a tree or in a field if you're desperate for a nap.
Don't underestimate the importance of getting to the start.  You'll be more likely to ride something that's easy to get to, and will appreciate it even more when you have to make your way back home afterwards.


A few comments on the ride descriptions you posted:


Asparagus &amp; Strawberries:  not much information posted, although the climbing (2600m) looks reasonable; being close to home is a plus.
Irish Mail: sounds like fun, hot food &amp; sleep stop at 345km, easy navigation on return route, but lots of climbing (5600m).
LGL / Poynton 400km: mentions easy night navigation, 24h services; bonus points for the name.