Can you send a link? These people list products but not prices and will not respond to retail enquiries
http://lead-crystalbatteries.co.uk/ They state that all products can be bought from Elfa Elementenfabriek, but when you go to their website their lead crystal link takes you back where you started.
These people show both products and prices for Galax-E, but "cannot supply for the time being."
http://www.motorcaravanning.co.uk/shopuk/leisure_batteries_lead_crystal.htm
An extensive Google did not show me any other suppliers for suitable automotive lead crystal.
Powerlite seem to be focused on replacing starter batteries rather then leisure, so a comparison with lead crystal is pointless as they are aimed at leisure batteries - high Ahr values and little requirement for CCA. The smallest Lead Crystal Galax-E for sale is 60 Ahr is listed at £148 inc VAT, but they do not quote CCA, although it should be comparable to an equivalent lead acid. Until they do I would not consider one as a starter.
.
If I understand correctly you bought the standard (not PAC) 20 Ahr version at 630 CCA and £209. That would probably be a bit too small for my D2 TD5 with the extra electronics over a Defender and I would be going for the 30 Ahr with 800 CCA at £250. But neither will last very long running anything with the engine off.
A suitable lead acid comparison would be Halfords Yuasa HSB013 62 Ahr and 620 CCA - £112 retail, (£88 with a Trade Card) comes with 5 year guarantee. So I guess if the powerlite lasted around 12 years it would break even against the lead acid. Powerlite are certainly much closer to a sensible economic case. No one is doing the same for leisure batteries - for 100 Ahr deep cycle the comparison is stark.
Lead Acid - TB25MF 110 Ahr from Amazon £80
Lead Crystal - Galax-E GLX-E - £248 comes with a 4 year warranty (if you can buy one)
Lithium - NX AML9138 - £859 could not see a guarantee. Cheapest I could find in UK from here;
https://www.allbatteries.co.uk/lithium-iron-phosphate-battery-lifepo4/12v/100ah.html