When it comes to DNA ethnicities the paper trail is virtually meaningless. There was a lot more migration all over Europe than we realize. For example, there are people from Great Britain that come out with a very large percentage of Scandinavian and very little British due to heavy Viking settlements in some areas.

You also have to look at the ranges on the ethnicities. It is impossible to pin down exact values. Some markers appear in several populations. It can be difficult to determine which ethnicity a given marker really indicates. Your could be much more or less than 88%. Your parents could also vary substantially.

Are your Dad's results DNA or just a paper trail? Over 5 generations, it is possible for some ethnicities to drop out of your DNA completely.

By random recombination, you could have gotten most of your parents Great Britain and very little of their other ethnicities.