i'm a uk player who has had similar issues. Is hard to quantify when stuff like this happens. I've always felt we have utterly trash routing to the EU servers, but nothing we can do - it's the ISP's and general infrastructure in the UK. I live in London, and it routes me to some strange places before taking the trip to Europe. USA is even worse - it routes to Amsterdam first, then the USA. I have people in EU who get better pings to USA east than i do, whom are 500 miles further away. Makes so much sense when the UK has a direct line to new york...
Aside from that, I'd recommend an OS reformat and reinstall of cs, don't copt over your config.cfg, do it again from scratch, but do copy over your autoexec. After that it's likely all snake oil, but:
Try HPET: ON in bios (hpet off always makes me have problems seeing enemies quickly. Then Run "bcdedit /deletevalue useplatformclock" in an elevated CMD (right click, run as admin).
Read up on MTU settings. If with BT, it's not normally meant to be 1500. http://www.richard-slater.co.uk/archives/2009/10/23/change-your-mtu-under-vista-windows-7-or-windows-8/ - can also change mtu setting for some routers - use same setting.
If you've used any network optimization stuff (like TCP optimizer), put it all back to default. Nto sure either way if these massive changes to network settings do a thing, but we can all assume most pros don't use them, so I've stopped now.
Get DPC latency checker and look for any crazy spikes. On some Intel Gigabit cards, any driver past 19.1 has intermittent latency spikes up to 2000u every 5 seconds, and should not be installed.
Try reinstalling GPU driver, and try the 347.09 BETA (if nvidia): http://www.techspot.com/drivers/driver/file/information/17658/
Clean mouse and mouse sensor, perhaps wash mouse pad if it's getting a bit groggy (QCK+ needs a wash every now and then).
Consider some new mouse skates for your mouse if you've had it longer than a year. Takasta on Ebay sells some ok ones, Tiger Gaming Brand for a lot if mice.
Like i said, this all could do absolutely nothing and could be snake oil, but it's everything I've tried at one time or another. Truth is, I too feel like i have issues in the UK with general lag problems, like I have to react to enemies like a machine or they will kill me instantly. Even recently switched from Virgin to BT in an effort to gain a more stable connection (i do think my BT 40/10 is much more stable than the 160/10 i had previously).
I also felt that the update that switched some client side stuff to server side caused me a few issues, too.
And finally, sometimes a genuine hardware/software issue can affect your level of play so badly as you try working out wtf is wrong, but then that period of time where it all went to hell can affect your confidence so bad, it can take a few weeks to get your mojo back, and you continues to play shite. Might be time to reformat, make sure it all working fine, and take a bit of a break.