There about £499–£599 if you pre register before and wait I have two kits and they work superb
They're not bad if you can get one for £500, but that's with the ridiculously small battery, which is going to be very over-stressed and won't last long.
It's possible to make something similar, using a handlebar mounted bag for about £20, a Q100H motor plus KT controller kit from BMSBattery for about £350 and a 20-cell 6Ah battery from Aliexpress for around £100. Say £500 for everything. That gives you a better more powerful motor, a better controller and an easier supply of cheaper spare parts. The work to install is about the same as a Swytch kit. Swytch customer service isn't that good, anecdotally; and you have to wait a long time for their stuff, while as it's only 2 weeks to get the stuff from China.
Their new kit is more or less the same as what I described, except that the bag goes wherever you choose to fit it and they use inferior parts to what I suggested. Also, when you take the DIY route, you can tailor the kit to exactly what you want.