New bike

Six and two threes really, Boardman if you want to commute on it or use towpaths etc, Allez if you fancy being racier. If you go for the Allez, worth noting that the 2019 Allez Sport (better groupset etc) is on the Specialized website reduced from £850 to £680, making it only £30 more expensive than this year's basic model and considerably cheaper than the Boardman.

I don't know an awful lot about the Boardman bike (other than they tend to be well specc'd and good value) but I entered the world of road cycling by buying an Allez sport (sora groupset) and can confirm it is a brilliant bike. I have the 2014 version with the curved top tube and external cable routing that gave it a washing line appearance; I think the latest models are even better with the dropped seat stays likely to give it a bit more comfort (plus neater cabling).

Anyway, the frames on them are great places to start. I upgraded the wheels and put a Tiagra group set on mine and it is still my good bike (apart from the commuter bike it is my only bike). Best upgrade though was a change of tyres as the stock ones weren't great.

Also, people seem to love the Decathlon offerings so it may be worth looking at them.

Allez was my first road bike some 15 years ago and it still has a place on my roster as “back up winter bike” - a very sound entry level bike.

As well as the more relaxed riding position of the Boardman you need to also think about whether you want disk brakes and the extra range that sora provides over Claris? If the answer to both is “no” I’d favour the Spesh

Boardman has a better overall spec than the Allez and will be less of a shock to the system coming from the mountain bike world. More relaxed riding position and disc brakes (albeit cable operated, which might catch you out if you've got hydros on your current bike)

From my relatively ignorant yet recently researching perspective ... it depends on your preferred riding style. The more road bike you go the more stretched out and horizontal you get. If you're intending to go off road at all or perhaps cycling plenty of hills ... the Boardman might be your best bet. It'll also depend on the particular model. Others on here far more experienced than I am can give you better advice.