To break it down you need, 2500 per month, lets see your ave 3 bed is £10.00 which is about average give or take £1-2 on area
You need 250 monthly custies at £10 each
re reading your post you are not starting till march next year, thats do able, just aim to find 5 houses a week at £10 , but target one reasonably compact area, clean them at weekend, you may have to explain it may take you 6 weeks to get round them while you are still employed but once you have enough, leave and do 4 weekly....
5 a week = 20 a month , you will be nearly there by next year
Its the cleaning you are gonna struggle with, 1st cleans etc, only thing i can think of if you cant do it because of work is sub it to someone for say 80 a day, even if you make nothing at least its getting cleaned for when you start