I don't know that I would use a thermometer inside the crockpot while cooking. I would cook the meat until it is almost ready and just start taking its temperature from there. You don't want to open the crockpot lid very often as that makes the heat go out and it will make whatever you are cooking take longer.
Thermoworks.com has some great thermometers that go on sale often, especially their POP thermometers, that take an accurate, almost instant read temperature. I have one of the more expensive ones, but the POPs go on sale for about $25 every couple of months. The advantage to these is that the instant read means it won't take but a couple of seconds to read the temp, the sensor is in the tip, and the long probe will reach far down so that you won't have to open the lid to your crockpot very wide, thus keeping most of the heat in the crockpot. These thermometer are certainly worth the price.
Thank you for your suggestion of a POP thermometer. I’m going to see if they are available in the UK.