Well I think this is like any other subject: you mileage may vary, and much may depend on what you can buy for parts and at what price. If you can get cheap good quality components AND you have the tools and mechanical knowledge to install them, then it's worth it. But let's face it, that last sentence is hardly a revelation.
IMHO usually you are better off buying another bike. The cost of replacing individual components just isn't a good value 9 times out of 10, particularly if you are talking drivechains. As someone else pointed out, it's very easy to get carried away -e.g. if you find a bargain set of wheels, you find they might only fit an 8 speed cassette, then you need new shifters, then you might need a new rear derailleur..... really, it's very easy to get sucked in.
For an fyi, attached is a pictue of my refurbished Raleigh SuperCourse. Apologies, but prices are in US dollars as that's where I live, but it should give you an idea.
Basic cost of bike: $220 (Wouldn't normally buy a used bike for this much, but it was a bit nostalgic for me as I grew up close to Carlton so I couldn't resist when I found this on Craigslist.org in a Boston suburb in Massachusetts). The bike came with a fairly new 36h wheelset. The original drivechain was worn and not shifting well. All prices include shipping.
Upgrades:
*Mudguards: VeloOrange Stainless steel: $50 off Amazon
Cane Creek Brake levers: $30 off Amazon
Shimano RSX Crankset and rear derailleur and bottom bracket: take off from another bike, but I'd guess you could find them on
Ebay with careful searching for $100
SRAM chain: $25 off Amazon
*Shimano Clipless pedals: again a take off from another bike, but you can get them on Amazon for $35
Sun Race 7 speed index down tube shifters: $20 from Amazon
*Kelly Take Off shifter mounts: $30 from Ebay
Koolstop brake pads: had them already, but can be got for $20
7 speed Shimano freewheel: $15 from Amazon
* **Retro pump: $15 from Amazon
*Retro backpack: $20 from Amazon
Cables: had them already, but approx cost is $20.
Handlebar tape: $15 from Amazon
Handlebars: traded for a stem I already had, could probably find them for $10
Stem: had it already, but I see them for $15 on Ebay.
Rear derailleur adaptor: $10
*not 100% necessary, admittedly
** looks nice, acts useless -you can pump up to 50-60 psi max
Total cost assuming you don't have any parts: $650 -which is about 405 quid -and we haven't talked about racks yet. So you can see it soon adds up -keep in mind this was with careful shopping of parts. However, since I already had some parts, the real cost was $470, or about 290 quid, so even that is a good amount (though granted if you have the bike already and choose not to use Kelly Take Offs or put cheaper mudguards on or the pump or backpack, the cost can easily come down to $175 or about 110 quid.
But that's the point: it's
very, very easy for costs to fluctuate wildly to make it worth your while (or not). Also keep in mind I do all bike mechanics myself which could be a different hidden cost. On the positive side, I do adore this bike -super smooth and gives me a bit of nostalgia too.