This isn't as easy as I had hoped.
I can't find anyone with UK stock and looking on the likes of AliExpress etc it seems that very few of the stores will ship from EU to the UK (good old Brexit).
But I am just getting more and more confused.
I am settled on a rear hub configuration and the Mxus Xf08c looks like a good contender, using a downtube battery with built in controller. All of the kits are shown as including a separate (silver box) controller but no battery. Are there two types of downtube battery, one with built in controller and one without? Or do I just order a kit and a battery and then not use the separate controller?
You can get batteries with the integrated controllers from Topbikekit and BMSBattery. There are advantages to the silver box type controllers with block connectors compared with the integrated controllers with waterproof connectors.
1. It's virtually impossible to test the sealed connectors to see which component is faulty if ever your bike doesn't work. it's dead easy to test every component when you have the block connectors.
2. The wire length is fixed with the sealed connectors, so you always end up with excess wire that you have to try and hide somehow, which can look very intidy, whiles as the aluminium box type controllers would be installed in a box or bag where you can hide lengths of excess wire.
3. there are more options for getting different powers with the aluminium box type, while as the integrated ones often only come in one version.
4. Some of the integrated controllers are quite long, and you really need a fixing that end to hold your battery to the frame. Without the controller there, you can drill a hole where you want. That's not possible with the controller in there.
I would always choose the aluminium box type because it's easier to test and there are more options to get what you want and it leaves the possibility to use whatever battery you want mounted however you want. Even now, the batteries that go with the Mk1 hailong integrated controllers are becoming harder to find, and it's very confusing because there are now several types that all look the same, but are completely un-interchangeable.
Controller in a a bag along with puncture kit, emergency lights and some other stuff: