There isn't a one size fits all answer. Sadly you will need to do your own arithmetic. If you pay 40% tax then the same car deal will cost you twice as much as if you only pay 20% top rate. In addition your employer may be prepared to give you a lease car at £300 a month but if you ask for the £300pm to buy your own car he may tell you to Foxtrot Oscar and give you a smaller amount. You may be forced to buy/lease a newer car with heavier costs than you would choose, as your employer may not want you showing up at clients in a 10yo Astra. Worse, your employer may have a ban on you showing up at clients in a Porsche 911, should you choose to buy one with the allowance. The mileage rates will vary too, as Rich points out above.
I've a mate who works for the NHS, he can get something like a BMW X3 hybrid and his tax bill is well under £200 a month. A new car, upmekret model, taxed, insured, ready to go, under £2k a year. Who wouldn't say "Yes please"? Conversely I run my own business, last year I billed clients for 20k business miles in my own car. That's 10k at 45p, 10k at 25p. £7k revenue, tax free, straight through the business and into my pocket. Fuel came in at just under 40 mpg, so about £2500, leaves £4500 to tax, insure, maintain and buy/lease the thing. Easy. I can have a £300 lease if I want or buy outright and I'm still in front. I currently run a ratty Mondeo, I should move up the food chain but can't be bothered. I actually run the Mondeo and the Mazda sports car on the mileage charges. I should get rid of the Mondy and track down a nice respectable workhorse, like a 5 series diesel with a recent-ish plate and a few miles up, that would tick a few boxes.
As to what you should do, you need to get the rules and then either post them all up here or do some sums yourself.
Edit - there are a few tasty cars that are low BIK. It used to be diesels, now it's hybrids. It's all a scam to keep new cars churning through the UK car industry, so play the game and get a hybrid. If your employer doesn't offer you a cash alternative then I wouldn't bother asking for one unless you have specific car needs, which you don't.