Worcester Bosch 42CDi or Slimline Gledhill unvented cylinder

[Benboiler]

Hello. New to the forums. Hope you can help. I have to replace an old gravity-fed system in my house (a big 4 bed, with 2 bathrooms containing 2 baths and 2 showers). Currently approx 18-20litres/min flow rate, but with old lead pipes being replaced may go up slightly.
I'd been going to replace the current Vaillant 25kw system boiler with a Worcester 42CDi combi, then thought I should go the unvented route, keeping the old boiler, which will apparently do the job.
Unfortunately my airing cupboard won't fit a Megaflow 250 unvented cylinder, which is what my (very helpful and patient) plumber recommends. It will fit a 210 litre Gledhill slimline unvented cylinder (475mm diameter).
Gledhill don't seem to get much love on plumbing forums, and my plumber isn't much of a fan. Also slightly worried the cylinder will be undersized. The only way I can get a Megaflow in is to move a wall in my bathroom.
So... am I back to square one? If a Worcester 42CDi can run two showers at once, then I'd be fairly happy, and would certainly prefer not to have something else in my house that can go wrong, and which needs an annual service.
Or should I persevere with Gledhill? Are they any good? Or should I just spend another grand and knock the wall down? All expertise gratefully received.

 

[flameport]

Can't see why you would want to remove a perfectly good boiler and replace it with a combi. 2 showers from a combi can work, but the performance will be far less than from a cylinder. It also means that when the boiler breaks, you have no showers or hot water anywhere until it's repaired.

If a Gledhill is not wanted, then other cyllinder manufacturers are available.
OSO Slimline RI or a Range Tribune Slimilne for example.

Any of those are not likely to be a problem regarding capacity - even for 2 showers 210 litres of hot water is a substantial amount, and also remember that the shower doesn't actually use 100% hot water because a considerable amount of cold water will be blended in at the valve.
You can also be reheating the cylinder while using it.