Your LBS pay retail property business rates (in some areas, that shed loads of money).
Wiggle pay industrial storage/distribution (or similar) business rates that are cheaper.
Your LBS pays retail/high St rents (which in the south east can be eye wateringly expensive).
Wiggle's rent will be for some out of town vast steel barn on an industrial estate, rather cheaper than the high st (per unit what ever unit you measure)
Wiggle etc buy huge quantities (possibly direct from Asia, so by passing the UK distributor at the same time), so will get large discounts.
Your LBS will buy small quanities, so no big discount.
Your LBS stuff will come in a card board box with some instructions, because the supplier (mostly) only supplies it in a box.
Wiggle etc often supply OEM parts in a sturdy plastic wrapper (as intended for factories to use). Some wholesale suppliers to LBS supply grey imports in OEM packaging, and are cheaper than the "official" wholesaler.
The on line prices you mention are pretty much the whole sale price your LBS will pay when ordering those parts. So you are not being ripped off by your LBS.
When caught short with a technical emergency requiring diagnosis/repair/advice, you LBS can help (if they have been supported enough to survive), wiggle can't help you a jot.
Interesting point about a man in a van doing "shop" repairs. A new one to me, I guess not a bad idea in areas where the level of repair business doesn't justify a full time mechanic, also means the shop owner just pays the van man an invoice, without the need for payrole/PAYE/NI/holiday pay/sick pay to pay, as that is for the van man to sort.
If you still wonder why local shops are expensive, the system is stacked against them.
If you want to make good money in the retail trade, then other areas have much better margins/mark ups that the cycle trade. People seem to complain that the LBS doubles the cost of a component, but happily pay $400 for a Iphone that costs $8 to make in the factory in China.
Always be wary of expensive quoates out of context. £150 for chain/cassette/rings. Well, if 9s and using middling quality, chain/cassette £50, assume 2 chainrings, £40-50/pr ish if Shimano, £90-100 if TA, that's £130 - £150 of your £150, leaving you £20 - £0 for labour.......... I would guess labour should be £25 - 40(?). If it was just "chainrings", then TA is most of the bill, £50 labour might be rather steep, but you never know if an extra "needs a new gear cable" so some such had been forgotten by the customer.
Sometimes it is worth mentioning the option of a new bike, just to put the repair cost in perspective. Then the repair seems more reasonable!