The water will be pretty hard as you're in Suffolk. DI only would be a cheap start up to make pure water but you would really need an RO system if you want to get into it. Not everyone persists with window cleaning. If you're not sure whether you want to do it for the longer term, why not start with DI to see how you get on? DI only would only give you around 2000 litres per sack (around £75) at 250 TDS and you would need two DI vessels ideally. If you get the DI vessels, they won't be wasted as they could go after an RO unit if/when you get one.
Omnipole do a 30 litre backpack on a trolley that looks pretty good at £395 (I think).
Then some 25 litre containers at whatever they cost.
If you want to get into it in Suffolk though, there's no getting away from an RO and storage tank with transfer pump I'm afraid even if you don't do it straight away.