The only difference being that once you complete an MPL course you will only be allowed to fly the type of commercial aircraft you were trained on
That is not quite correct.
The last phase of the MPL course is a full type rating conducted by a TRTO (in this case Flybe) as part of an approved MPL scheme. Once the type rating has been completed, including LST landings, an MPL licence will be issued by the authority. There is no requirement for an individual to fly with the TRTO beyond that point, other than that they will no doubt have been bonded for the type rating. An MPL licence is just that - restricted to flying in a multi crew environment. Should an individual wish to do another type rating, after licence issue, they may do so. They just need to complete another type rating course. What they cannot do is a type/class rating on an aircraft certified for operation with a single crew member. The majority of airline pilots do not keep either single or multi piston land ratings current and are therefore operating using an MPL. The only difference is that, should they wish/need to do so, they could revert to single crew operations or fly a light aircraft for leisure.
As in a fATPL, at 1500 hours the MPL becomes an ATPL, with the same multi crew restriction.
Contrary to opinions stated earlier, I think it will be a no brainer for the airlines, who will be pleased to have a hand in the initial training.