It certainly is subjective, and because it is, recommending what someone should buy, can be tricky, as it might not always turn out to be the best recommendation for them.
If someone with 2.1/5.1 speakers says that he/she is looking for an improvement, but is considering stereo and isn't too fussed about deep bass, then it's a lot easier to steer that person towards bookshelf speakers + amp; or maybe studio monitor speakers.
If that person though, likes their 2.1/5.1 speakers and wants the similar again, but better. Trying to steer them towards stereo speakers, because the sound is better to those doing the recommending, may not always turn out well; such as the example above.
It's hard to know whether stereo speakers and an amp, will be a good move for you. You might find the sound is fine and a subwoofer is not needed; but judging by your question above though, about will they have the same oomph as 2.1 speakers, I'm guessing you would want a subwoofer as well. Now, you could add one afterwards, but unless you get a really good used bargain, you'd probably end up paying £100 at least for one. If it's new, even then, £100 for a subwoofer is cheap.
I think ideally, anyone with 2.1/5.1 PC speakers that have been recommended getting stereo speakers, it would be a good idea to listen to some at a shop like Richer Sounds. At least, that person would have an idea whether pursuing that route is worth their time and money.
If that's not possible, then you can only go on other's recommendations and opinions. Certainly the safe bet is to get what you know. On the other hand, if you do go with stereo speakers + amp, and maybe a subwoofer at a later date, it could turn out to be a good investment.
If you'd prefer to stick with 2.1, then the best bet is Acoustic Energy Aego M. Acoustic Energy are selling refurbished units themselves on the famous auction site for £90, or you could buy a new set for £130.
Harman Kardon Soundsticks III are also good for £100, but the looks do put some people off.
Logitech or Creative are fine for cheap 2.1 speakers, but with £100 to spend, there are better options.