I’ve never tried those and can’t really judge on the actual absorbency of the product. It says on the pack that those Boots slips are similar to the Attends Regular M8 (M stands for Medium) which I’ve already tried and has a pretty descent absorbency level (2099mls ISO absorbency).
Maybe you can try the Attends Slips Regular M9 which is the next level in terms of absorbency. (2899mls)
Sample-wise I know that Tena’s pretty good at offering free samples but most Tena products are more expensive than any other brand out there.
Let us know how you get on if you get to try those Attends M9.