Redundancy Selection - unfair?

The shop floor team consists of 5 employees working 42 hours per week (full time), 3 employees working 35 hours per week (part time) and 6 employees working a variety of other part time hours (25 and below).

We are in financial difficulty and need to cut costs by reducing staffing levels and want to keep the 5 contracts at 42 hours, reduce the 35 hour contracts from 3 to 2 and reduce the other contracted hours to just one 18 hour contract.  This justification is based on the operating hours of the store and the rota that covers those operating hours. The changes will save the company about £20,000 per year.

For the redundancy pool we want to only include employees on the 35 hour contracts and below on the basis that there are no changes planned to the number of 42 hour contracts and therefore none of the 42 hour posts are redundant.  My thinking with this is that if they were to be included in the selection pool the 5 people on 42 hour contracts will claim those 5 posts as "suitable alternatives" anyway, and the other staff could not because the contract varies in terms of contracted hours so the 42 hour contracts for them would only be an "alternative".  My understanding is that legally we have to place people in suitable alternative posts first and look at only alternative posts second.

Is this understanding correct?  And if so how does this get around part time workers discrimination because in effect only the part time workers would go into the selection pool because the number of part time posts are reducing?

Thanks.