Hi !
Good point from Paul there make sure you find that out from the person in your company looking after this. Company cars 'usually' make sense for larger cars than the Toyota Yaris. The Yaris is in the super mini class of cars which makes it more suited for private sales than to run one as a company car. that said, if you research out everything and indeed it's good for you to use the work scheme then certainly consider it.
One of my best mates has the same option as you, get a company car where they do everything, or buy his own and they'd provide running costs etc. He went and got the car he wanted and took the money provided for running costs - he was stipulated to buy something 'suitable' and 'professional image looking' as well as it could only be up to 5 years old.
So you may get stipulations like that as well!
I'd definitley recommend Diesel - for mine and my bro's everyday cars we've had Diesel's for the last couple years, and from now on always will do so long as the economic benefits are there. If petrol was to ever go down by a good 20p a litre I'd switch back, but thats not likely to ever happen in overtaxed and expensive UK, so Diesel it is until maybe the newer fuels have more of an impact.
Lastly, I'd recommend you buy a used car than a brand new one, unless you Ms/Mrs moneybags and can take the 20% depreciation hit as soon as you drive it off the forecourt!
take a look at the large car supermarkets around the country, they all advertise in the whatcar magazine you mentioned, and that magazine also covers alot of the financing options your considering.
Good luck ! any more questions stick em up here and will answer!
Cheers! DtM! West London & Slough UK!