The key resource is *money*. I moved to London from southeast Ireland about 6 years ago, doing a very similar job here to the one I did there, and surprisingly found the cost of living here (rent aside) to be more or less the same - but I earn about £10K more here than I did there. Now consider that when I lived there, it was entirely normal to see most retailers dealing with media like DVDs, CDs and games keep products at their launch price for their entire lifespan. So many games remain €60 games, CDs remain €25 cds, and DVDs remain €30 DVDs. Over the last 7 or 8 years, a bunch of those retailers wondered why so many consumers in Ireland collectively told them to shove it, and started shopping online. But many of them failed to adjust their prices accordingly (Hello HMV, hello GAME) and have unfortunately since gone bust, which is bad for everyone. But to blame the consumer is a shortsighted and incomplete conclusion which fails to fully account for the root cause of the problem.
But let's shift focus to the UK.
The UK national average salary based on the most recent census is about £24600; knock off say 30% for tax, that's ~17200. Knock off say 8400 for rent, that's 8800 left - for travel, heating & electricity, food, insurance, and discretionary income.
Now say to someone "That record shop down the road will sell you a great album for £10, but Amazon has it for £7.50". Or "The specialty DVD shop has that film you wanted for £15, but it's available online for £5 + postage". Or "Gamestop has the game you want to buy, but it's on Steam at 75% off". Because for a massive chunk of the population, those are the choices. The numbers are pretty simple, but the context is not.
Amazon vs the bookshop is a good example of this - I've used Amazon for a good while and find that as far as online retail experiences go, they set the standard. I also have a very real problem with their attitude of not paying any tax in the UK, and have for some time been making an effort to limit how much stuff I buy through them if it's at all possible to get it elsewhere. In particular, I make an effort to regularly visit actual bookshops (including the small one a couple of streets away from me) and spend money on whatever they've got in stock that looks interesting (rather than seeking out specific titles). I like them, and I figure I can afford to spend more on the same books there and the "extra" cost is worth it for the convenience and pleasure of having them available (I've had my bacon saved on more than one occasion for late-purchase gifts, for example).
The point I'm trying to make is that it's not quite as simple as saying "support local retailers if they're available" - because for a lot of people the cheaper prices offered by online shops are the difference between "I can afford it" and "I can't afford it". Some of the time, people don't realise what they lose from not going to a local shop - but other times, the local retailer simply isn't offering a compelling or realistically priced option.
Consider the way that the Hugh Fernley-Whittingstalls of the cooking world will exhort people to buy everything locally and on the day - which is awesome if you actually have the time and option of going to a locally-supplied greengrocer, butcher, baker, fishmonger ect - but a bit of a fucker if like many of us you work 8 or 9 hours a day and have to spend 1-2 hours a day travelling to and from work. There's a reason that the supermarket concept gained traction in the first place.