"I only wanted it to browse and not for downloading anything"
Browsing is downloading, anything you see or hear from your computer on the internet is downloaded.
"also do these tings work on a similar basis to a pay & go mobile phone"
Yep, just like that, you buy a top-up, you have the interenet until it's used up then you buy another top-up
The main thing is to check that whatever providers dongle you go for has decent coverage in the area you will be wanting to use it in. all of the main providers have online coverage maps.
O2
http://www.webmap.o2.co.uk/
Orange
http://search.orange.....html?channel=direct/
Vodafone
http://online.vodafon...bile_network_coverage
T mobile
http://www.t-mobile.c...overage/street-check/
(I really should bookmark those pages so I don't have to search for them each time :))
Keep in mind when you check coverage you are looking for 3G coverage as that is mobile broadband, if you will be in a 2G area then you will get very very slow download speed.