I have a little bit of knowledge on this 🙂 I work for the largest UK manufacturer of bricks, so we supply our fair share of slips. I haven’t dealt with the company you have mentioned, but as far as I can tell they just buy bricks from others and cut them into slips. There are actually a few of our bricks renamed on their website. The ones you are interested in aren’t made by my company. Ultimately the brick slips they are selling are just a cut brick, so quality should be pretty consistent from whoever you buy them from. For a small quantity their prices look to be okay.
Let me know if you want any more info.