Ah, that makes more sense!

Take a look at Section 5a of Bedfordshire's Academic Regulations,
here. Paragraph 6.10 explains the two different ways they calculate your final degree classification - either using only the credits from both the penultimate and final stages of the programme of study, or just those at the final stage. As you're doing a top-up degree from a HND, it seems clearly they they'll just used the credits form the final state - i.e. those you actually gain at University of Bedfordshire. They calculate your final degree classifications as follows:
"6.14 The Honours classification is based on the average of the marks attained in 90 credits at the final stage, including at least 30 credits from the designated project."
As you're doing 120 credits this year, it means they ignore the marks from one entire 30-credit module (as long as it's not "the designated project"). As a result, the capped 40% will have precisely zero impact on your overall degree classification. So just ensure you average 70% or above in the other three modules!

Finally, note that the bottom of page 3 and the top of page 4 (within Section 5a - link above) list quite a few degrees which
don't follow the "normal" regulations in some way. So check that your course isn't on that list. If it is, let me know and I'll check how the rules for that course vary from the norm.