I can't stress enough the importance of riding as many bikes as you can before you decide. What suits someone else may very well not suit you, this goes for the Triban, the TDF, in fact just about all bikes.
I know
Halfords don't tend to allow test rides

, but i don't know about
Decathlon. That said most Decathlon stores are big enough to ride around inside anyway
It may be worth your while going to a large chain such as
Evans Cycles. They carry wide stocks so you should have a fair few to go at and see what fits best, unless your very tall like me in which case no-one ever stocks larger frame sizes
As for 'other gear', make sure you know how to change a puncture and get the gear to do this at the roadside, a pump, inner tubes, tyre levers. I also carry a chain tool and a missing link, but these are things you can build up over time as you get more into things.
Clothing wise I'd say comfort first, if you're happy to wear lycra, fill your boots, if not then don't.
Helmets are a very contentious issue. I wear one, many don't.
Clip in pedals are also a love hate thing. I can't ride without them, but if you haven't used them before I'd recommend you get the bike first, get some miles in, then once you are totally comfortable with the bike may be consider going clipless
Above all make sure you enjoy it.
Good luck and ride safe
