Something that has little outlay, that you enjoy doing, and that has a high earning potential.
The high street is still dying with rents & rates staying fairly high and councils not budging on the planning constraints to ensure empty units get filled, so consider on-line as the way to go for people to access your business.
Either a service which is in demand now, or a product which is in demand now.
Best quick cash for short term would be a stall on a car boot or market selling a reasonably priced product at reasonable price to punters, but at very little cost to the seller i.e plants - they are in demand now and if you grow them yourself the outlay is more time, than money. The income won't be huge, but the profit margin will be.
Or,
Get yourself to the auctions, pick up cheap, but good quality antiques which are transportable, and aim to sell to the London designer crowd - better profit. (could sell on ebay but don't do 'collection only' if you are oop north, allow for courier as most London buyers will have a fortnightly nationwide collection round). The outlay will be higher, and the turnover less than plants, but the income will be greater if you pick the right antiques at the right price.
Failing that, open a pop-up car wash - they are always packed! :-)