So in Germany most cities and towns have the elevation on Wikipedia (example: https://de.wikipedia.org/wiki/Kalkar)), I like to add these information to osm, as it is quite useful while cycling. Is there anywhere I can find the elevations of British towns (and maybe other POI)?
Unlike many European cities there is no tradition of stating a towns elevation in the UK. I would recommend NOT to do this for the following reasons:
Stated elevations elsewhere will usually refer to a specific benchmark level: probably most usually based on the town hall or other admin building (e.g., Denver State Captirol).
OSM nodes for places are not (generally) in the UK attached to such locations and therefore an ele tag could be highly misleading (i.e., the elevation tag does not correspond to the lat/long of where the node is placed). This would change the semantics of the ele tag.
There are many other more useful ways of acquiring the same data: OS Open Data elevation models, spot heights etc. See what cycle.travel & OpenCycleMap do.
If you must do this, please do not use the ele tag: instead use a modified form of the tag, s.g., place_ele or similar.