What is the safest way to adapt a two pin British shaving socket to a European two pin (Germany) for use with an electric toothbrush bought in the UK?

The first step to safety is dimensional compatibility.

I just tried my UK to EU adaptor. It seems to fit nicely into the 230V slot of my UK shaving socket. So chances are, your plugs might be able to work into EU's socket. It may also depend on the type of power plugs you have. For example, this type of electronic brush power plug seems to work: https://www.amazon.co.uk/BENSN-Replacement-Electric-Toothbrush-Charging/dp/B07NQ5WW6Y/ref=asc_df_B07NQ5WW6Y/?tag=googshopuk-21&linkCode=df0&hvadid=534908806066&hvpos=&hvnetw=g&hvrand=12293407434938819159&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1006598&hvtargid=pla-1402678432936&th=1

However, I tried my Oral B electronic brush charger in Portugal. It doesn't work! But it turned out that my UK to EU adaptor not only accepts the three-leg plug, but also hosts the Oral B brush two-leg plug nicely.

sometimes the "shaver socket" in Europe is slightly different than in UK (more narrow). Sometimes not. So it's a matter of luck.

If you want to use your toothbrush and the socket is different you can buy an extension cable and connect to a socket in the room with an adapter.

If you can't find a UK->EU adaptor that will take a shaver plug (as in the top answer), you could also use a combination of a standard UK shaver -> UK Type G adapter, then a UK to EU adapter. This is slightly less safe than using one single adapter, but much better than trying to force something into the wrong socket.

This seems to be a bit of a mess.

The UK shaver plug looks similar at first glance to a Europlug, but it's not compatible. The pins are thicker, spaced closer together and rigid instead of flexible. Most UK shaver sockets and shaver adapters will accept Europlugs but most European sockets will not accept UK shaver plugs and trying to force them is not a good idea.

Historically travel adapters sold in the UK for travellers going abroad would typically be designed to accommodate the UK shaver plug as well as normal UK plugs. Unfortunately they also often used 4mm pins rather than 4.8mm, this let them fit into Italian and soviet sockets but left them a loose fit in French/German sockets. They also often had questionable earthing arrangements picking up only one of the two side clips on a German socket (not that it matters for a toothbrush charger but it just indicates general sloppiness to me).

In 2016 BSI introduced a new standard BS 8546 for travel adapters in an attempt to improve the quality/safety of them. I can't find any evidence of an actual legal requirement to comply with said standard but major retailers certainly seem to have adopted it.

Unfortunately a side affect of this seems to have been the loss of the ability to accept shaver plugs from most travel adapters sold at retail in the UK.

So that leaves you with a few options if you want to use your current charger, none of them great.

1. Stack a shaver adapter on top of a travel adapter. This means you can use high quality plug type specific travel adapters but a stack of adapters means more leverage placed on the socket which is not ideal.
2. Compromise on quality/approvals of the travel adapter to get the ability to accept the shaver plug directly.
3. Change the plug on your charger, the trouble is it's not possible to change it back if you want to use it in UK bathrooms afterwards.

The other option I would seriously consider if you are travelling a lot is to get a Euro-spec charger for your toothbrush. As I mentioned above most UK shaver adapters and shaver sockets will accept Europlugs so this gives you more flexibility for little downside.

I had a spare Braun charger, from a previous toothbrush. I just purchased a European spec 2-pin plug (about £2, but it took some finding), and replaced the UK shaver plug. I wrote EURO on it with a Sharpie, and keep it for travel. Works perfectly, no adapter needed, and no compromise on safety.

The safest way without a doubt is getting an EU charger. Same with laptop chargers -- just swap the IEC C7 AC cable ("figure eight") or IEC C5 ("mickey mouse) for a cable appropriate the local power socket. It doesn't get safer than that. Avoid plug converters whenever you can.

Most UK to European plug adaptors have specially shaped Line and Neutral holes that will allow either a shaver or 13 Amp plug to be used in a standard European mains socket.