When the time to move out comes, the time to clean comes with it. People living on rent are obligated to clean their lodgings before moving out by their rental contracts. Moreover, they have insured that they'll clean their lodgings with a rental deposit. It will not be returned to them, if the place is not in the same condition in which it was when they hired it.
First of all, pack your stuff and take them out of the place. You must have more space to operate.
See if there are any damages in the property and fix them. If they are beyond your abilities, hire a specialist. Have in mind that stained carpets and upholstery furniture are also considered major damages, so if you can't remove a stain hire a professional cleaner.
Start cleaning the place from the top-down, removing the cobwebs and mould first.
Proceed with dusting the furniture and polishing the wooden pieces.
Vacuum the floors in the entire lodging. It will be best if you have the carpets professionally cleaned, especially if you have old stains on them, but it's your choice.
Clean the stains from the kitchen appliances and the room itself. Wipe the floors and counter-tops, using a proper solution for the purpose.
Clean the other rooms, including the bathroom, too. Do not skip anything, because it might cause you your rental deposit.
And as a proper final, clean the windows, aerate and deodorise the place. You can use lemons cut in halves, they absorb all bad odours and leave a pleasant citrus aroma.
And if this seems too much work to you, skip it all, hire a professional move out cleaning London company to do it for you, and go to have fun with your friends or family.