It is unlikely you will be able to find any French andouille sausage in a regular British butcher's shop, unless you can find one who is prepared to make it for you from scratch.
Your best bet is frenchclick.co.uk, who import andouille from Brittany. There are similar sausages available in specialist stores, however.
If this isn't to your taste, try the smoked toulouse sausage, which you can order from redonline.co.uk or wealdsmokery.co.uk.
Finally, there's the Italian nduja sausage, a very heavily spiced pork salami from Calabria, described as similar to the andouille sausage. You can order some from natoora.co.uk.
You could always try making your own andouille sausage of course, and this YouTube user will show you how:
