It depends on what you want, do you want a generalised tracking device, which will need to be removed every so often to have their batter recharged or do you want something that is basic fit and forget?
Then there is the question of how does the tracker communicate with the outside world, some of these devised only uses Bluetooth, great for finding keys lost down the back of the sofa, but so not much use if you are not within a few metres.
Obviously with a bike you are going to want something with a SIM card. Here you can safely ignore the 3G, 4G or 5G abilities of the SIM all that is required here is the ability to send a text message with the the lat and long of the device at any given moment in time (it ain't trying to stream uncompress video), so good old 2G GPRS is fine.
Here is where using a device with a non UK SIM has an advantage, because it won't be tied to any of the four UK network, it will simply talk to which ever network is providing the strongest signal. But herein is the bind, due to the gross stupidity that is Brexit, we don't know what the mobile phone roving situation will be beyond the end of the year, so a €3 (£2.69) per month contract might become rather more in January, if somewhat unlikely to increase beyond £10 a month, so interns insurance and of peace of mind in knowing where a £5000 e-bike is, not a lot, but still an annoyance.
There are a couple of e-bike specific trackers on the market and at least one that has an Edinburgh dealer who will fit it for you as part of the purchase price.