Sterling Airlines were one of the first MPL schemes and when they went bankrupt the fairly fresh 737 MPLS were taken on by another 737 operator. So to say the MPL isn't transferable isn't strictly true. While the essence of the above post is largely correct low time/fresh MPLs have been reemployed by operators of the same type in the past. Likewise Monarchs MPLs were taken on by Easyjet. However for Flybe's MPLS the issue is of course there are no other Dash 8 operators in the UK and very few in Europe.
There is nothing in the easy access rules that directly states a second MPA type can't be added to a MPL prior to achieving a full ATPL, but I suspect the necessary amendments to the Part B and Part D is what stops most operators considering other MPLs with minimal to no line experience from defunct airlines.