There are a multitude of 'visa application services' which promise to get you a visa to almost anywhere but the vast majority of them are simply scammers who charge you a fee for what you can do yourself anyway.
However VFS Global (as in 237SJ's link) provides the OFFICIAL visa application service for Russian visas. (i.e. the Russian Embassy in London has outsourced the process to VFS Global and it's VFS Global's website that you'll be redirected to if you seek out visa information on the Embassy website).
Start here
http://ru.vfsglobal.co.uk/
Click 'Private' if you'll be be visiting family or friends. Click 'Business', 'Special Events' or 'Tourist', as appropriate, otherwise.
Note that, while the application form is available online, attendance at a VFS Global visa centre (London, Manchester or Edinburgh) is compulsory.
Some of the questions in your post are answered on this FAQs page:
http://ru.vfsglobal.co.uk/faqs.html
Email
[email protected] for further information or call 0905 889 0149 (M-F, 0830-1730, 55p per minute on top of your normal call charges).