You can change the profile to 'road cycling' and it will avoid paths with bad surfaces.
Not sure about the UK but I've cycled quite a lot around here in Europe (12 countries so far) and from my experience, if you don't follow the official bike trails you're missing a lot. They are usually nice scenic and quiet roads with good (asphalt) surfaces. I'm talking about trails like the Alpe Adria Cycle path. There are tons of them in mainland Europe.
But you're right - if the surface is bad, then it doesn't make sense to follow the trail. I will add a feature that will highlight the selected surface on the line (similar to what Komoot already does) so you can better investigate the route and make sure the surface is alright.
I'm also planning to add a feature where users could rate the track so you could filter out the bad tracks based on other people's reviews.
Avoiding the official bike trails in Europe has advantages. For example, when you plot your own routes away from the touristic ones, local people might be more willing to chat or offer you a place to stay for the night because they have never met a foreign bicycle traveler before. Also, bikepacking is big these days, so a lot of cyclists want routes that don't have good asphalt surfaces.