Your best 'value for money' bet is something with a Chinese Honda copy (which are widely accepted as reliable these days) and a known pump for which spares are available. Generally this means Interpump (best) or AR (Italian..🙄. but then you run a multione! [so do I🤣]). There are brand new Honda copy 13hp machines out there for around £600, but it will most likely be the pump that fails and I have no idea how easy spares would be to find. Certainly on eBay there are a lot of 'returns' being sold.
CRYTEC COMMERCIAL 13HP Petrol Pressure Washer 3625PSI / 250BAR Power Jet Cleaner
WWW.EBAY.CO.UK
CRYTEC WX1300. Crytec - The Petrol Washer Market Leader. Crytec Ultra Durable Pump. The red is for blasting, yellow...
I have two, both with 13hp engines and one with a 15l/min pump and the other 21l/min at higher pressure. The 21l/m is far superior for cleaning. Once you have sufficient pressure (which is not as much as people think unless you are into abrasive cleaning etc) then flow to carry the dirt away is way more important. You may as well go petrol, as even the biggest electric machine (unless three phase) will seem as effective as pissing on it compared to a proper 13hp machine.
You'll be better off with an IBC than a water butt. 15l/m will run ok off most decent mains supplies, 21l/m usually needs a buffer tank anyhow.
For your budget, you will need to be looking secondhand but they are out there. Both my machines are Jetmac, and they are well put together. I picked the Loncin 15l/m one up for £350 needing a bit of work (and have fitted new pump pistons annd unloader valve, arouind £200 total), and the Honda GX390 21l/m was a total bargain at £700 barely used. It's belt driven too and is considerably smoother. I now keep the Loncin one on a pallet to make it easy to take one to site if I need one.
Comet are a 'known brand' of pumps, although usually at the domestic end. This might do if buying new-
13HP Loncin Petrol Industrial Pressure Power Washer Jet Wash 3000PSI 21 LPM
WWW.EBAY.CO.UK
This power washer would be our premium industrial Loncin range, perfect for farms or industrial use. 13HP Loncin 420...
Personally I'd go secondhand, and be prepared to fiddle. They are not that complicated. Whatever you do, don't skimp on flow rate. You'll never go back from 15 or even better 21l/m, and time spent washing is dead time.
Learn about nozzle sizes too- look up nozzle charts and get the correct ones, test it with an inline pressure gauge, and try a nozzle slightly larger for less pressure if washing delicate paintwork (rather than relying upon the unloader valve and sacrificing some flow rate)
Edit- this would be excellent. The same as I have only Loncin rather than Honda and AR rather than Interpump.
https://jetmac.co.uk/shop/ols/products/loncin-petrol-cool-drive-3000-psi-21-litres-a-minute-jetwasher/v/g420beltAr211-300-PSI
Half size option that would be reasonable (and in budget) but trust me, you'll never regret the extra money spent on flow rate.
7 Horse power Loncin Ar 200 -12 pressure washer
JETMAC.CO.UK
Loncin G210 7 horse power petrol engine with low oil alert as standard Commercial Grade Annovi Reverberi triplex pump...