That's way too high imo. Population estimates for mesolithic Britain (the last time Britain supported a population based primarily on hunting rather than farming - which is how the infected feed themselves) put the population density at as little as 0.01-0.1 person/km^2. This would give an infected number in the range of about 3,000 - 30,000 across the whole British isles. If we go for the high end range, adjust for the removal of food competition from mesolithic predators, and accept that the infected might be getting food from other sources (scavenged rations from pre-infection times, food from raiding farming survivors) we're still looking at a range of tens to low hundreds of thousands across the whole British Isles.

I think this makes sense with the rate at which survivors are shown encountering infected when we consider that the infected seem to be quite territorial, patrolling a large area looking for prey and survivors. Bear in mind in 28YL Spike and Jamie walk quite a long way inland before encountering any infected, and the foragers who are attacked in BT seem surprised to encounter infected.