an avr (audio video receiver) is a the center of any "real" surround system. basically it is a dac, amp and source switch in one. the benefit of these over one of those htib (home theater in a box) setups with the dvd player as part of the speaker amp is that you have much more flexibility and you will not run into many of the problems (like not being able to get 5.1 input on a 5.1 output system!) that htibs may have.
soundbars have come a long way. originally they were meant as more of a replacement for tv speakers (which are mostly awful) yet not a true replacement for a decent home theater setup. nowadays there are some rather high quality soundbars on the market (as well as much mediocre stuff as well.. its not all great) and some even emulates (to a certain degree, its not perfect) a surround sound sytem. what they are good at would be fitting into a situation where normal speakers are not possible as they get mounted just below a tv and require nothing else.
surround vs stereo? that is your decision. either will work depending on preferences getting a nice pair of bookshelves from a big brand and an amplifier might fit your needs perfectly (klipsch icon's, pioneer bs22, etc..) or perhaps getting active speakers (micca pb42x, rokkit 5, mackie cr3/cr4, etc) would work for you as well. you can always add an active subwoofer to such a setup (dayton makes low priced yet fair sounding subs for people on a budget). some people who have more than one or two sources pair such a system up with a stereo receiver instead for more connectivity and greater output power (its hard to finda good cheap yet powerful amp other than a stereo receiver).
my personal setups...
5.0 klipsch quintet iv satellite system, 450w klipsch subwoofer, pioneer vsx-30 receiver. i use this on the main pc which gets used for movies, music, light gaming and general use. very crisp sound and i greatly enjoy it. not all sources of audio are 5.1 (music, windows sounds, youtube are not) but it certainly is enjoyable for those that are. since its multi-source (ps2, ps3, pc, ipod, laptop, etc) a receiver helps with that.
2.0 hk thirty tall bookshelf speakers, dta120 amplifier. this gets used on a secondary pc. simple setup with one source so just an amplifier works well. no subwoofer needed as the big bookshelves have enough bass on their own.
2.0 sony floorstanders, sony 5.2 receiver. these get used on the main living room tv. while not as 'crisp' sounding on treble as my klipsch, the bookshelves were a great buy (i picked up a pair for under $150/pr). the sony receiver was also cheap yet has worked well to power them.
so, why avrs? quite simply you will not find many amps better than the dta120 for any reasonable price. there are some 'junk' brand amps you can buy (you cannot believe their rated specs) that advertise more but those arent worth buying. any 'good' amplifier with the rated power or greater of many receivers will cost much more than just buying a receiver. it seems separate amps are more a part of the 'high end' audio world.
i'm not sure about europe, but the monoprice 10565 knockoff or the original energy take 5.1 classic take sets with a budget receiver are a decent entry level system. (set $250-300+/-, receiver $100-150+/-)
for only 50gbp however i think your choices will be very much limited and you often get what you pay for. if you are more than happy with budget products than that is your own choice and you are lucky. my ears can not stand budget level products anymore after hearing better.