The 'spare parts' section of the UK Worx website seems to be utterly useless.
Looking elsewhere suggests that the WA3152 is compatible with model numbers WG150, WG250, WG540 and WG541 ONLY, whereas the WA3156 is compatible with a different range of models.
It would seem to make more sense to pay around £54 (inc shipping) for a battery that you
know will be compatible:
http://www.ebay.co.uk/itm/Worx-C1817A005-18V-NiCd-Powerstation-Battery-Charger-New-For-WA3152-WA3127-/370885505883?pt=US_Power_Tools_Batteries_Chargers&hash=item565a7fb35b
However, in the first instance, it would probably be even more sensible to phone the UK product support helpline to see if a battery can be sourced from within the UK. You probably bought your product from Argos, Amazon, Homebase or B&Q. If so, phone 0845 202 9679. (If you bought from worxtv.co.uk phone 0871 225 5616).