I really fancy some Rothaus Tannenzapfle...

Can it be purchased here in the UK?

I'm sure it will be. You could try asking them for stockists via email.
http://www.rothaus.de/html/11kontakt/11_kontakt_fs.htm

If you're in London this places claims to have it.
http://www.kurzandlang.co.uk/menu/

http://www.beersofeurope.co.uk/acatalog/Beers_of_Europe__Rothaus_443.ht ml

Beers of europe are great. I once ordered a huge case of beers for my brother for xmas and they sent me the wrong case so I rang them and they said to keep it and they sent out the other case too