What I'm missing in many responses is any information about how safe some countries are to cycle in. I rode the Dunwich Dynamo quite a few times, and did LEL once, but I would never consider picking the UK as the destination for a relaxing cycling holiday. Likewise Poland.

The UK has lots of cycle paths look at the Sustrans web site. The circle of East Anglia on mybikeguide is pretty good if you want flat.

Basically the mybikeguide.co.uk has a lot of info and links to sources of info country by country.

If you want super safe and super flat then nothing beats The Netherlands or Denmark though Germany comes a close third. But if you want warm as well then parts of France and Spain, Italy come into it. Transport of bike or bike rental becomes an issue as does a little local language to chat with mechanics etc.

I did/do both. I have cycled many times in Denmark, along the coastal road running north of Copenhagen, past the beautiful areas of Charlottenlund and Klampenborg etc. That entire coastline is beautiful with white sand beaches and views towards Sweden. At Helsingoer, some cyclists take their bikes on the ferry to Sweden and cycle there. Past Helsingoer (where my in-laws and OH are from), there are cycle paths into forests and lakes, to Jørn Utzon's house, Nakkehoved lighthouse close to Gilleleje on some cliffs and lovely village of Hornbaek. You can't marry into a Danish family and not cycle.

But there are many other places in DK to cycle and the article in the Guardian has more info on that than I do, eg Bornholm and Jutland.

There is alot of cycling in Sweden too, for example along the Göta canal and yes I have been there, it is similar in parts to the Thames Towpath, which yes, I cycle parts of it regularly to get to the shops.

I cycled in East Anglia. Have never before been yelled at so many times and nearly forced off the road a couple of times. Not something I want to repeat. The Sustrans network is a joke.

I cycled TCR twice, so I know about mountains. Thanks.

It's fair to say that the Sustrans network could be better but it is no joke. It fits a need. East Anglia generally, don't know. But the route on mbg is pretty good.

My husband and I cycled (parts of) La Velodyssée in western France. While France isn't the most bike-friendly country (we also cycled along the Loire and in Alsace, on sometimes dangerous D-roads), this route was great and mostly on bike-paths.
https://www.cycling-lavelodyssee.com/

And, as menachem says, the entire Netherlands!

Intrigued by alluring's reference to The Cornish Way [which despite over 20 years in Cornwall I'd never heard of] i had to look it up.

https://www.cornwalls.co.uk/sports-a...ornish-way.htm

As some of the terrain it seems to cover is pretty hilly [sadly no map that I could find except a misleading graphic which suggests it follows the coast, which clearly it doesn't as there is a footpath that does that] I would suspect that it's not flat. But it should be pretty safe.

I've ridden most of La Velodyssée and it is very flat I found it a bit boring, but certainly pretty enough. Generally D roads in France are pretty safe for bikes but worth sitting down with a map and maybe even street-view to check on traffic density while planning.

Yes, it's flat. Flat is perfect for me. My husband goes with his team when he wants to do cols in the Alps or the Pyrenees.

I beg to differ.

I thoroughly enjoyed cycling around the Bodensee (Lake Constance) and also along the Mosel between Koblenz and Trier. Both trips were with rental bikes, which were in darn good shape. Another "excursion" , albeit a fairly short one, (48 km round trip) was south of Bolzano through apple orchards and partially along the Adige River.