To me the idea of purchasing a soundbar is to improve the poor sound that pretty much all LCD slimline tv's have. Too be honest, £100 is not going to get you a really decent one. I would use the money that you would have been getting the Apple tv with and put it together with the money you would be getting the soundbar with. At a later date get the Apple tv.
I have a Yamaha soundbar, this one:
http://www.johnlewis.com/231604280/Product.aspx
I paid around £220 for it, but the sound quality is fantastic. The good thing with this soundbar is that you don't need to find additional space for a sub woofer. I'm really picky, but I find that there is no need for an additional sub woofer, has the one it has built in is superb. You can adjust the sub woofer level on the soundbar ive mentioned via its remote control, i have the sub woofer on mine turned to maximum, and find its more than enough for me, explosions in films have a real punch. I can't recommend this soundbar enough.
Yeah you won't get the sound quality of a seperate 7 speaker surround system from a soundbar, but to me, soundbars are for people who either don't have the room for a 7 speaker surround system, or they don't want to be running cables around the room. I got a soundbar because I didn't have the room. If you get a good soundbar, in my opinion it's not a compromise on sound quality unless you are an audiophile.
Re connections:
The Yamaha soundbar I've mentioned has a coaxial input for STB (sky+, freeview box etc), optical output for BD (this is where you connect a DVD, Blu ray player etc), then there's a optical input for TV. You can just connect the tv directly to the soundbar, but I done this with my tv (which is 2 years old), but I found the sound quality was good, but not as good has when I wired the devices into it separately.
If you do run out of optical ports on a soundbar, it's possible to purchase a one in, 2, 3, or 4 out optical switchers, so its no big deal if you need more optical ports and you have none left on your soundbar. I got a good one from Keene Electronics for around £50 has after I'd got my soundbar I got an Apple tv and needed an additional optical port to connect it to, the optical switcher comes with a little remote that allows you to switch between the connected sources.
Here's a link to it:
http://www.keene.co.uk/electronics/multi.php?mycode=AUD41
There's a review on the John Lewis website for the soundbar, which gives a bit more info
http://www.johnlewis.com/231604280/Product.aspx#BVRRWidgetID
If you need anymore advice, just ask and I'll see if I can help