I've heard people say
Halfords don't allow test rides though.
That is correct. My wife wanted to try one of the bikes and they only let her go up down the crowded bike isles in-store, they said "once you take it outside it is classed as 2nd hand goods and we are not allowed to trade 2nd hand goods." Pretty dumb considering the policy of most other bike shops which do let you have a ride.
Decathlon is similar, in that they let you ride in-store but the isles are so wide and long it is enough to give you a rough idea of bike fit. But having said that, my wife bought a rockrider from there, used it for a few short rides and decided she hated it so we took it back for a full refund no problems what-so-ever.
The Carerra TDF has a double-chainset, which to new comers to cycling would probably be horrible to use as much more fitness is required to tackle hills with a 52/39 chainset. The Triban 3 has a triple 30/39/50 which is great to get you up them steep hills, but once you have sort of progressed from new-comer to medium fitness levels you will find that you wont even use the small 30 gear and then it just becomes dead weight.
Horses for courses.