Just did a search for you, the animal magic near me used to have bigger tanks have you tried them?
Animal Magic
4, Maple Walk Chelmsley Wood
Birmingham West Midlands B37 5TS
Tel: 0121 770 8133
May be worth a call
Alternatively :
Edgewood Aquatic & Pet Supplies
94, Edgwood Rd Rednal
Birmingham West Midlands B45 8SG
Tel: 0121 693 6005
Finns Aquaria
232, Warwick Rd
Birmingham West Midlands B11 2NB
Tel: 0121 753 0162
Aqualand Pet Centre
861, Alum Rock Rd Ward End
Birmingham West Midlands B8 2AG
Tel: 0121 327 0846
Its a big list, there must be one that sells the size of tank you want
I have no experience with any of these so can't recommend them, I will say that an animal magic I used to go to was very good and the guy knowledgeable but thats just one shop and the one in the list for all I know could be useless