The issue with the external thermostat is also the external thermistor on a cable because this thing needs to be firmly secured in the water at all times.
Pulling it out accidently without notice during maintenance can result in a cooking tank, because it will measure outside the tank and keep on heating. Then if you want a fail safe it's still best to have a heater element with an internal thermostat anyway. If it happens for what ever reason, maybe it's not you but the cat pulling the thermistor out then it still will switch off in time.
I've been in this hobby for decades and can only speak for myself and must admit over the years it happend a few times that i forgot to unplug the heater during a water change. I guess i'm not the only sleepy head around who did this. It also once happened to me with a stainles steel heater, then also it keeps on (Over)heating and it did melt the seal. After i filled the tank with water again i felt a tingling sensation when sticking a finger in the water and touching ground. This stainless steel heater was leaking power and just not enough to blow the fuse. That was my first and last stainless steel heater, these things can be dangerous for sleepy heads when leaking water and electricity after overheating and melting the rubber seal and not showing the damage.
If this happens with a regular in tank glass heater and it overheats the glass will crack and the heater will be broken, blow the fuse and or burn out the heater element.
Personaly i would recomend an ordenairy old school glass in tank heater, simply to always be on the safe side...
No idea about all available products in the UK, but brands like Sera, Dupla, Colombo, Eden etc. all have them all are standard and equaly good all the do is heat... I found the heaters from Eden pretty affordable and good quality...
Simple and easy to use Internal Aquarium Heater from Eden. Eden is a brand from the prestigious Oase manufacturing group, well known for premium pond products. It provides accurate water temperature readings thanks to its thermostat system that reacts only to the water temperature and not to the...
www.aquatix-2u.co.uk