does it say in their agreement that they will source any bike or does it say any bike they normally stock? check the small print.
for £650 your not really saving that much on the tax, personally i think you really need to go up to the £1000 to make the tax thing worthwhile (although i do agree its handy having it come out monthly whatever amount you have). But if you are not happy with halfords, cancel the whole thing and go and find a bike you want on a reasonable repayment plan. some places may to interest free credit. nothing worse than paying for a bike you're not really happy with IMO
for £650 your not really saving that much on the tax, personally i think you really need to go up to the £1000 to make the tax thing worthwhile (although i do agree its handy having it come out monthly whatever amount you have). But if you are not happy with halfords, cancel the whole thing and go and find a bike you want on a reasonable repayment plan. some places may to interest free credit. nothing worse than paying for a bike you're not really happy with IMO