Not sure that a self measured mail order suit will come back as 'superlative', or for that matter better than something off the peg.
At least buying in a store will give you the chance to try on a few jackets, and dont forget that a nearly perfect jacket could theoretically be adjusted (depending on the work needed) by a local tailor.
My advice would be not to go with a self measured suit. While taking the required measurements is not necessarily that difficult, it is something which someone with experience can do far more effectively than someone without.
And dont forget that there are a great deal of nuances in posture or body shape that you will find very difficult/impossible to identify.
I can appreciate the desire to get a quality suit whether your 15 or 55, as a (well) tailored suit looks infinately better than something off the rack.
However, for the Â£349 that Blackstone Tailoring offer a 2pc merino wool suit, you would be advised to spend a little bit extra and get a MTM suit from one of the good travelling tailors.
Have a look at:http://www.jasperlittman.co.ukhttp://www.rakehell.co.ukhttp://www.nortonandtownsend.co.uk
There are also a few cheaper options (and certainly not of the same quality):http://www.dress2kill.comhttp://www.kingandallen.co.ukhttp://www.mandbt.co.uk
Unfortunately I dont know whereabouts you live, and the tailors I mention are based in the south-east. But regardless of where you live, have a look around and see what a local tailor could do for you in terms of made-to-measure.
If you do decide to go with a self measured suit, I would strongly recommend going to a local tailor and have him measure you. Perhaps buy a tie off him as a gesture. Any tailor worth his salt will be pleased to see a 15 year old lad entering the world of tailored clothing.
Good luck with your suit, whatever you decide to do.