That's exactly it, gun-teching is no way to make money.
People whinge about £15-20 labour for a spring change or r-hop installation, but when you consider that a shop likely has to pay their tech £12k+ a year (add on NI contributions, pension contributions etc) at the low end and suddenly you need that person to be making absolute minimum £40 a day to break even.
Custom guns built to order too are another pit, airsofters in general seem to be obsessed with the idea of a 'package' deal, no one's selling you a holiday! a G&P M4 with a load of extra bits added on costs the sum of it's parts and probably a bit of labour for putting it all together, you're not gonna get money off the cost of those parts when the margins are already small! As for what I would like in a retail store with a web-outlet;
Consumables: Reasonably priced quality BBs - note, not CHEAP, reasonably priced. These are probably going to be your bread and butter product as anyone can buy a gun and keep it for 10 years... they still have to buy BBs for it though! Always-available reasonably priced pyro - this is where you can offer people 'bulk deals', £3 each for pea grenades or 4 for £10, your margins will be relatively small but you're guaranteed to shift them in big numbers as long as you can keep up with demand!
If you've got a CQB site nearby start selling blank firing grenades and blanks to go with them, again your margins will be smallish on the blanks but it's a consumable item people will be back for.
Gas - Abbey gases and WE Nuprol seem to be the gases of choice for most airsofters (with good reason), you can make some money on these as the wholesale costs are surprisingly low!
LiPo batteries and chargers - people are gonna buy these, might as well buy them from you!
PARTS!!! Get a good range of regularly broken bits; springs, tappet plates, hop-rubbers, inner barrels (less so, but everyone wants a tight bore for their new gun... you can even professionally fit it in store for them for a small fee!), motors, deans connectors (again... fitted for a small fee).
Big-ticket items:
A range of good-quality starter guns (combat machines anyone?) which you can sell as a starter kit with a cheap charger and a NiMh battery. Chuck in a Sansei style mask (they're SO cheap) for nothing and they'll fly out of the door.
Optics, torches, grips - airsofters love bolting shit onto their guns! Get a decent range of attachments going on and set them up near the guns, impulse buys are good for business
Gear/nylon/etc I can't really comment on, I haven't bought anything retail from the UK for ages so I don't know what the market is like, or what people tend to buy.
The NUMBER ONE most important thing though; customer service and a welcoming environment. Best airsoft shop I've ever been in is Pro Airsoft Supplies in London, staff couldn't do enough to help me and I didn't even say I was buying anything, they straight off the bat said "if it's on a wall rack feel free to have a play with it, if you want to shoot anything, let us know and we'll get a battery and some BBs". The customer service bit for them comes in when there's an issue, they honour their warranties (I expect mainly because they don't sell cheapo double eagle/SRC/etc guns and don't get many returns!) and do so without quibble or fuss, same for incorrect orders, they sort it same day. That reputation is well known because people talk about it when they get really good service, but not as quickly as when they get bad service!
Wolf armouries on the other hand... I went in, spoke to the guy and said I was visiting London and was in the market for a new AEG (pretty much guaranteed him a sale!), but getting guns out of boxes for me to have a look at seemed like too much hassle and when they did come out of the boxes he was pretty reticent about letting me actually hold them... no sale there and I won't be going back!