if you can you'll be better of going to an independant retailer, they usually sell a greater range of kit - Scarpa and La Sportiva boots are built on a southern European foot shape rather than a British foot shape, Meindl and Raichle are built on a northern european shape and Hi-Tec are built around something vaguely foot shaped but of indeterminate source...
i would seriously reccomend saving up the pennies and then getting to Ambleside and Keswick, both towns have 5 or 6 decent retailers who carry a wide range of stock and you'll be able to mooch about getting the boot that fits at the best price.
Fishers, Neddlesport, Rathbones and Outside in Keswick come well reccommended, as do the Climbers Shop, Rock and Run, Gainers and Stewartsons in Ambleside.
most of these 'serious' shops won't do forces discount - they subsidise the local MRT's - but if you're buying a high quality boot you'll be paying at least Â£90 anyway and they'll last you years. theres no point you buying a boot based on price anyway, the one that fits best is the one you should buy - anything else is a waste of money as you'll throw them away in a few weeks.
Blacks staff often have very little training and no experience and Field and Trek - while one of the better chains - have gone into administration and closed half their stores with no warning to the staff. they turned up on monday morning to find the shops empty of stock and locked with a note on the inside of the window telling the ex-staff where to get their P45's.
I buy a fair bit of Hiking equipment, and these are the discounts I know of to HM Forces-
Snow and Rock- No discount
Tauntan Liesure -10%
Cotswold camping- 10% (more if Bulk purchase)
Blacks - 10%
Field and Trek - 10%
Karrimor - 10%
Mountain Equipment - 30% if you contact them direct or at least used to be www.peglers.co.uk - 20%.If they do not stock the item you are after they will order it for you.