2.5mm t&e ok for immersion heater?

[SUNRAY]

Did you miss this?

A good tip to ensure long term reliability and performance from your brush is to discharge the brush completely every 6 months and then recharge it fully. Such activity helps ensure a long battery life.

Or this?

It is less common today, but in the past, leaving the brush on the stand, whilst connected to power would wear the battery out quicker, causing what was known as a ‘memory effect’.

Perhaps you did not read this far:

If you do want to really try and gain the absolute maximum from the battery, it is best to charge it fully and then discharge it almost entirely (not completely), down to say approximately 20% then recharge it.

Taking this approach ensures you get fuller charging cycles, rather than letting the battery discharge to 75% and then keep topping it up.

https://www.electricteeth.co.uk/charging-an-electric-toothbrush/

Well NONE of that text or anything like it appears in the MI's for our 2 electric toothbrushes. Both advise permanently storing in the AUTOMATIC charger.

However it seems you are being very selective in your quotes in order to prove your factually incorrect posts or perhaps you didn't read far enough as you did not quote this from the link you posted:
In most instances, manufacturers do suggest it is safe to leave your brush on the charging stand at all times. If you take this approach it is unlikely you will ever be in a position where you are out of power.

 

[SUNRAY]

It's not fit for purpose...seen too much of it disintegrate with age/heat.

I don't think there's much to chose between high temp PVC or rubber, PVC goes hard and brittle and Butyl breaks down but both should give at least 10 years of service. My preference is butyl and 2 flats of mine on E7 [so daily use of immersion heater] have not been replaced since mid 90's.