Optical zoom is the only thing that matters. Digital zoom is the same as cropping the picture afterwards. All it does is crop to the middle of the picture. That reduces image quality because you are only using the pixels in the middle of the picture.
Digital zoom is a completely pointless "feature" for any stills camera to have. It is marginally useful on a camcorder because it is harder to crop a video afterwards, but it still reduces image quality.
Forget the concerts. Any camera good enough to get good quality images in the low lights of a concert won't be allowed in.
The quality of the image is determined by sensor size and image quality. Any cheap compact will have a crappy small sensor. A DSLR or EVIL camera will be much better because of the larger sensor, but may not be what you are after for a travel camera, especially if you want it to be light weight.
Very long zooms tend to have rubbish image quality. Anything more than about 15x is worth avoiding if you care about distortion or vignetting or light falloff at the edges of the image. A lens with optical image stabilisation is worth paying extra for.
What sort of budget do you have in mind?
EDIT - I've got a Panasonic TZ-25 which is my backup camera. It's a good choice for travel. It is lightweight, well built, has enough zoom for most purposes, quite a good wideangle and reacts quite quickly. The lens is good (comes from Leica). Pictures are pretty good for a compact and they don't try to cram too many pixels onto the small sensor. It's not suitable if you plan on getting it wet or sandy.
The TZ-25 battery is a bit small and weedy so I keep a spare charged and ready to go. The spare battery was a cheap third party one from Amazon but it works well. I use a Sandisk Ultra 16GB SDHC card in it. That's quick and again reasonably priced at Amazon.
The TZ-25 is no better or worse than any other cheap-ish compact for concerts. It's not great at high ISO, but you won't find one that is. You can turn the flash off and it's small enough to get past the guards on the doors, so that's okay. There's plenty of manual control if you want to get into photography, but there's also a lot of useful scene modes. I was quite impressed with how well the sunset mode worked, which is nice on holiday!
EDIT 2 - The tz-25 is available for £117 at Argos at the moment: http://www.argos.co.uk/static/Search/searchTerms/5598964.htm
Add in a couple of 16GB class 10 Sandisk Ultra SDHC cards at £10 each: http://www.amazon.co.uk/SanDisk-SDSDU-016G-U46-Sandisk-Ultra-SDHC/dp/B007BJHETS/
Then add a spare battery for £9: http://www.amazon.co.uk/gp/product/B002AMP3Y4/
and you've got a decent camera kit for under £150.
(Note: There are cheaper spare batteries, but most get mixed or bad reviews).
If you want a case for the tz-25 as well take a look at the Lowepro Apex 30 AW, which might look expensive at £20, but it has an inbuilt rain jacket which goes around the case as well as plenty of padding and a pocket for the memory card. There is just enough room for camera, battery and spare card.