That nice Mr Google tells me that Boots sweeteners contain only saccharin (whereas other sweeteners might contain acesulfame K, aspartame, sorbitol, sucralose, stevia or xylitol). So it would seem to make sense for you to seek out an alternative which is also based solely upon saccharin.
The most widely sold brand seems to be Sweetex. Tesco sell packs of 1200 for £2.90. Packs of 600 are more commonly sold elsewhere, priced as follows:
Asda £1.89
Tesco £1.90
Sainsbury's £1.90
Superdrug £1.99