Technology allowance...?
The car allowance analogy is kind of flawed. At the start of this century, the car allowance morphed into a cost-savings dodge. It was used as an alternative to pay-rises post-tech-crash-and-9/11.
In the office I worked in at the time, it didn't change anything for the people who didn't already have a company car -- they would use their own car whenever practical beforehand, and claim mileage against the project, and they continued to do so. When impractical, they'd get alternative transport and charge it against the project. But that was in a Scottish office of a London-based company. Down at HQ, it led to the situation where people who would normally have used public transport to get about were told they had to use their cars, and it took them longer to get to many places. It reduced choice, thus reducing the ability to be flexible to specific needs, and thus hampered operational efficiency.
My last employer tauted the idea of a laptop allowance a year before I left. The general feeling among the employees on the discussion boards was that it was a Bad Idea.
Having personal responsibility is all well and good, but if we're all buying individually, the company loses economies of scale, and compatibility testing becomes impossible for even the most important core business software. Does a lowly helpdesk operative have to buy a second PC out of his own pocket if the ticket-logging software crashes on his machine?
Secondly, the coverage of support. For a mobile workforce, there is really only one place you would be able to guarantee UK-wide coverage: PC World. So in order to have somewhere to go whenever your PC breaks down, you would be squeezed into buying from one of the UK's most expensive suppliers, and getting your support from a company who happens to have an notably poor reputation for quality of support.
Until computers are reliable, consistent and 100% commoditised, companies will continue to need to buy direct and in bulk.
And lets face it -- that's never going to happen