Yes, you will need to run around the carpark for about 5 minutes, having done a test before, immediately after and then again 10 minutes later.
Unfortunately Salbutamol (eg Ventolin and other inhalers) is not approved for use when flying as it can affect reaction times. However, you can be on inhaled (not oral) steroids (usually the range of brown inhalers).
The AME's there are very helpful, but it will cost you in time and money if you fail that section. It's a good idea to chat to your doctor and start taking whatever is necessary asap.
Remember also that the carpark at Gatwick is right at the airport, so the avgas fumes etc are very representative, hence the carpark run and not a treadmill inside.
On a slightly different note, I have found a vitamin supplement that has been invaluable for me and has massively reduced my asthma and hayfever, so that I cruised through my class one earlier this year. However, I am a distributor (it was cheaper that way) so I don't want to put details here (too much like marketing!) If you want details, contact me directly.
Good Luck!