Preliminary graphs from an ongoing project with Pablo Beramendi (apologies for the very plain presentation):

For both plots, the loyalty rate is calculated as the probability that an individual votes for party x in election t given that they voted for party x in election t-1. These probabilities are frequencies taken from electoral surveys in each country.

In the UK there is no clear trend, but in Portugal it appears that loyalty is declining. However, notice that the two x-axes are different lengths (due to data availability), so it is difficult to say whether the trend in Portugal would hold up over a longer period.