I think it's a little harsh to refer to the NS1000's as Play.com specials. Play sell them, along with other sites. They are very good sounding headphones for what they cost. Sadly their build quality is their failing.
You are right though, one cannot expect £60-£100 headphones to sound as good as a speaker system that costs several hundred, or maybe over a £1k, depending on what is included in that figure.
You do have to spend a lot more to get the same quality from speakers, that a pair of good sounding headphones provides. £200 headphones are going to sound better than £200 speakers.
A lot of headphones are overpriced though, which makes price to performance ratio in comparison to speakers, harder to judge. We get ripped off in the UK with quite a few brands, compared to the US. If you take Grado headphones, we pay a lot more than Americans do. Higher prices are expected certainly, with Grado being a US brand, but some of them are nearly 2x the price here in the UK.
Then there's the possibility of an amp to consider. Some headphones work well without a headphone amp, some are wasted without one. If you're looking at headphones that do need an amp, then a headphone setup will cost considerably more with one, assuming it's a high end one.
If you're looking at headphones that do not need a dedicated headphone amp, then the headphones could be connected to a speaker amp (assuming it has a headphone socket), and will use the connected DAC.