dstiles - 8:44 pm on Oct 2, 2011 (gmt 0)
For UK use, note that IPs can be anywhere from /11 down to /22 or even /23 for broadband/DSL providers. Allowing a full /16 is likely to end in a mix of UK, UA, RU, NL etc.
Some UK ranges should in any case not be permitted. The fact it is listed as UK does not mean it's broadband/DSL: it may be a server farm, which should normally be excluded.
Also, in some cases IP by country can be a bit hit/miss: the latest allocations are not always included in country lists that may only have been in use for a year or less - sometimes as long as two or three years.
It is also quite feasible for a bad bot to originate from a compromised computer (ie it's got a virus and become part of a botnet). These can be on popular broadband ranges more often than server ranges, but do not rule out the latter: there are a lot of compromised servers around at the moment.
My personal view is that you have to define a range of header field conditions (not exclusively User-Agents) and IP ranges that are acceptable. If you are constricting by country that still does not mean you can ignore bad bots and scrapers. Remember that scrapers are often run by "seo" people who run from broadband lines, often in your own country!