Your best bet is frenchclick.co.uk, who import andouille from Brittany. There are similar sausages available in specialist stores, however.
The closest who are likely to get to Andouille sausage is the merguez sausage, a chili lamb sausage from Morocco, which is increasingly available in the larger supermarkets, such as Sainsbury's and Waitrose.
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: