It's rational. UK epidemic is not yet out of control. If you look at the region breakdown in UK the cases are very well geographically distributed, the highest concentration as of yesterday is Hertfordshire with only 13 cases, or London with 100 cases if you consider that one administrative unit. Some epidemics in Schengen are highly focused on specific areas which means the growth rate can be expected to be higher. It makes sense to consider Schengen as one unit as it has open borders. It would make little sense to ban travel from individual countries in Schengen.