Forget the mobile shop idea. The vast majority of people have cars these days (or shop online), so you'd have very few customers. (I recently supervised a survey team on a massive housing development in Didcot, asking people about their travel habits. One of the questions asked about how many cars there were in the household. Over 6 evenings of knocking on doors I only found two homes where the answer was 'none'. In response to a question about how they travelled to the shops, quite a few people said that they never did because they bought everything online).
Courier work doesn't pay well for most people. If you work alone it will take you a long time to build up enough customers to provide a decent income and, without being part of a network, you'll find it difficult to offer reasonable prices. e.g. if someone wants to send a parcel from Belfast to London it will cost them a fortune if it's the only parcel in your van and they have to pay for your time and fuel, as well as your profit, to get it there. However if you work, on a self-employed basis, for an established network (such as Yodel) the customer can pay a low price because you'll only be taking the parcel to or from a local depot but you then get stuck with having to accept the pittance that Yodel, and other similar firms, pay their drivers.
Garden work can pay quite well though. I had a friend who made a decent living from it until he retired. There are a father and son team who do some work for me occasionally, cutting back my front hedge to keep the footpath clear and a few other tidying up jobs which can be done with their trusty chainsaw. It typically takes them 10 minutes to do the main work, with a further 5 minutes to get all of the hedge cuttings, etc into their van. I pay £60 for that, having knocked them down from the £100 they were originally asking, and usually a bit more for an additional task or two which I'd like done. (They've got to spend time taking the stuff away and then paying to tip it but it still seems like a damned good money earner to me!).
One of the advantages of going into the rougher side of gardening (such as cutting vegetation back with a chainsaw or hedge trimmer, rather than planting out beautiful borders) is that it's fairly easy to identify potential clients. You simply need to walk/drive around an area, looking for overgrown gardens. (It's far less easy to find people who need someone to do a bit of wallpapering for them).
Some people who run burger vans (etc) seem to make good money from it. I used to do the PAYE returns for a guy who ran a big burger van, employing three staff. He had a couple of nice cars and a lovely home in the country, with horses in the paddock. Similarly the lady who (on her own) runs the van I use quite often seems to do fairly well from it. However both of those people are/were operating from good sites. (In each case they've got mains electricity and running water available to them, on sites where there are lots of potential customers). I've known other people who've given up trying to run burgers vans (or similar enterprises), either because they've been making no real profit from it or simply because they've got fed up of being hassled by the food hygiene inspectorate.