I have heard of ukcarbroker and from what I know are a good company.
The prices on the Audi A5 does seem very low indeed, please check when enquiring if these are pre registered cars and will you be the first registered keeper? Doesnt make too much difference if they are pre registered but if your comparing quotations you need them to be like for like.
As mentioned previously you could approach an Audi dealer with this price and see what they can do but I can help thinking they may throw you out of the showroom as this price is unbelievable.
There are other new car brokers out there to research like broadspeed, drivethedeal, broker4cars always worth checking as a comparison.
good luck.