The National Library of Iceland has had a similar program for a couple of years. The national TLD is collected three times a year and made available via the Wayback Machine
. The english version of the project's page
is rather terse, but according to the Icelandic version, selected pages are collected more frequently when warranted, e.g. political debates around election times. Icelandic law requires publishers to deposit copies of ther work with the National Library. This includes web pages so the library doesn't have to worry about copyright.
For a small country with few resources, co-operation with other small countries and archive.org is probably best. The task of collectiing the british TLD is orders of magnitude bigger. It may well be cheaper for the British Library to pay for a system tailored to their needs rather than figure out how to make archive.org's software do what the library needs.