You can get either a monthly subscription for £10 a month by direct debit or you can pay £199 upfront and the subscription will run for the lifetime of the box. Back in the day this was a bargain as after 20 months the guide data was 'free' but obviously as the boxes get older this becomes more of a gamble.
Periodically you get a discussion on here about buying a new lifetime which starts all the regulars going about if/when/how Tivo will relaunch in the UK or if/when/how they will stop the service.
Given you can buy lifetime subscribed Tivos on eBay for less than £199 I would seriously consider that option if you have yet to purchase.
If you have bought one then I would try a month on subscription and consider forking out the Lifetime if you think you'll still want to be using Tivo in 2 years time - pay with a credit card and you have some recourse to a refund if the service is stopped before you've had your moneys worth.
Tivo Customer Services
Phone number: 08702 41 84 86
Hours of operation: Monday - Sunday, 08.30 - 23.00 GMT.
The user guide and stuff is also here on the official Tivo UK site http://www.uk.tivo.com