If you have 1mb ADSL in London, you should be able to have your ISP up it to 8mb (nominally) without any problem at all.. I get 8Mb at home if the sticks. Bear in mind this 1Mb, if its ADSL, will only probably get 256kbps up the pipe, so big attachments etc. being sent may well stall things for a while. If you have a ‘proper’ connection of 1Mb each, you will almost certainly have an SLT with your supplier who should be able to syupply you with nice graphs of usage. Else, therte are plenty of broadband suppliers – plus.net have been very good for me for over 10 years.
However, my blind guess would be that the problem is more likely that you’re running Microsoft Small Business Server – which sticks AD, Exchange,ISA,DHCP,DNS and MS-SQL on one usually underpowered server. This box will be handling your email and proxying your internet access most likely, so if it’s suffering you’d notice – is file serving a little sluggish too? Printing? Have you only got a single server? Or none at all, in which case this is all hot air 🙂
If this is the case, I’d say you’d want a minimum of 4Gb of RAM in the server before you even look anywhere else – shouldn’t be too expensive. I’ve seen plenty of installations with 1 Gb on SATA hard drives, and frankly I’m surprised they boot up let alone work.