The cheapest sound bar I can find is the NEC Mitsubishi Sound Bar 90-
100000724 NEC Mitsubishi NEC 90 SERIES SOUND BAR BLACK
I'm not saying it is good, only that it is cheap.
Dell, also offers this Sound Bar 90 -
Dell : NEC NEC MultiSync Soundbar 90 - PC multimedia speakers - black : Displays : Home & Home Office
Yamaha, a more popular and better quality sound bar, is not going to be cheap -
Yamaha Soundbar Speaker. Digital Sound Projector. Elegant Space-saving Design. Hdmi In/out. Speaker
Yamaha Sound Bar - Compare Prices, Reviews and Buy at NexTag - Price - Review
Here are an assortment of other sound bars courtesy of Amazon-UK -
Amazon.co.uk: soundbar
I think the Logitech Sound Stage TX101 might be pretty good, but it is close to double your budget -
Logic3 TX101 SoundStage - 5.1 Surround Sound Home: Amazon.co.uk: Electronics & Photo
As to what is good in your budget,...well...nothing really. You might find something you like, something that will be OK, some thing good for the money, but nothing that is an unqualified 'good'.
However, there is an alternative. It has its drawbacks but it is cheap and easy. I've always thought (though never tried) that you could make the equivalent of a sound bar out of ordinary 'egg/pod' speakers.
You build a small shelf to set your TV on, then put the front and center speakers under the shelf, which is then in turn under the TV. Then you mount your rear speakers high and wide on the same wall.
The front/center speakers give you the direct sound for voice and general movies, and the high wide speakers give you a sense of space and special effects.
Since everything is on one wall, you don't have speaker wire running across the room for people to trip on or mess with. The front three speakers are securely under a shelf under the TV, and are not likely to be tipped over.
If your budget really is stuck at £100, then simply look around for a sound bar that you can afford that looks good to you. That's about the best you can do on that budget.
Also keep in mind that many sound bars are complete system, they come with built-in amps and connect directly to the TV, no AV Amp is needed.
The last option is the 'more than your budget' Q Acoustic QAV 3.1 system at £432 -
Q ACOUSTICS QAV 3.1 SPEAKER SYSTEM - available from Superfi UK Visit http://www.superfi.co.uk/index.cfm/page/moreinfo.cfm/product_id/4009 for details
So, you can find sound bars near £100, but few here are going to recommend any of them. They are what they are, they cost what they cost, they do what they do, if they satisfy you, then that's fine. If not, the only option is to spend more.
Steve/bluewizard