Good morning
I have a lot of support for SkipDivers view but the cronky old MTB may have gear ratios that don't fit well with your current and expected levels of fitness, especially for going fast on the road.
If you find that only the largest chain ring and smallest sprocket work for you on the road then rethinking the gearing would be a lot cheaper than a whole new bike.
Looking at the ADV 8.8 I would personally not buy it for road use, with a 48/32 chain ring and a cassette with 11-12-14-16-18-21- 24-28-32 sprockets, I would find it almost a four speed bike. There is nowhere I ride that the 32 ring would ever be used (Worcestershire), I would almost always be in 48-18/16 or 48/28 with occasional dabbles into 48/21.
The ADV at 10.5kg is not stunningly light and my own personal experience as a fun rider is that you can train away about 8lbs of bike weight in about 2-3 months and this costs nothing apart from some food to power the ride. Clearly as you get closer to the maximum possible fitness for you more training is needed to replace smaller weight savings.
A few years ago I would have said if its a Boardman then it will be a great bike for the price, nowadays I would look closely at the spec and the price. If it is reduced, is it really reduced as
Halfords seem to be doing "special offers" on Boardmans in the same way as they do with Apollo and Carrera.
However if you want a new bike then buy it because you want it not because it is the best possible use of your money. :-)
Bye
Ian