Yes millions. According to Asthma UK (
https://www.asthma.org.uk/about/media/facts-and-statistics/) there are over 5 million people being treated for asthma in the UK and asthma is not the only condition that will make wearing a mask impossible. Clearly GPs are not going to be able to produce anything like enough letters within 10 days.
According to the link that you posted, there are 3.6million adults in England being treated for asthma. So:
1) how many are shielded because their asthma makes COVID-19 too dangerous for them?
2) how many have to travel to work?
3) of 2) above, how many use rail or bus to travel to work?
4) how many of 3) above have mild asthma that doesn't prevent them wearing a face covering?
According to thie site:
https://www.cbre.co.uk/research-and-reports/our-cities/city-commuting-by-rail
about 2.9 million people use rail to get to work and about half that (let's say 1.5 million) use bus, and your linked asthma webpage says thjat 1 in 11 has the disease. That means that even if every person using bus or train doesn't use both, there are 4.4 million passengers. Even if every asthmatic travelled to work on public transport, that would only be 400,000 before those who didn't use public transport as grouped in my points 1),2) & 4) above. I doubt that more than 50,000 would be involved, if that.
Of course every one of them would be well acquainted with their local surgery to be able to get a standard form letter on their doctor's headed paper within 10 days.