There's also Kingspan and Telford. All much of a muchness really. Depends on your budget. Have a look at what the warranties cover and any exclusions.
Go into the national merchants and ask who makes their own brand cylinders. When i worked for a National merchant years ago, their own brand unvented cylinder was made by Heatrae Sadia, the same company that makes Megaflo cylinders. All of the warranty and customer support was routed through Heatrae, we just were not allowed to advertise that fact. So you could get a premium item for a budget price. Also as Teki said, regardless, check the warranty conditions, as some may be "return to base" warranty, meaning you (or your plumber) would have to take the unit out and return it to the shop rather than the manufacturer sending out an engineer to look at the issue. (and they dont normally cover the cost of the returning plumber)
Plumb Center (now trading as Wolseley) Quite a lot of their own brand products are / were (this was around 6 years ago so things may have changed) made by national brands, such as their own inhibitor was made by fernox.
Apart from the obvious, is there much difference between 200 and 250l tanks, i was told 250 be best, but height means a 200 is easier, joule short cylinders seem to be back order 4-6 weeks