How long are you planning on keeping the car? There are some excellent lease deals about if you change every 3 years, with an average £320pm over 2 or 3 years and 10k miles per year allowance (taking into account the initial payment weighting upfront).
You can't beat that with a 3 year purchase right now. If you get a £30k R for £27k and it retains 51% of RRP (quoted GFV for an R) then it will still have lost £11700 over 3 years, or £325 a month. That's without taking any interest into consideration. Chances are there'll be about £3.5k interest to consider if you put £7k down to finance the £20k balance - Thats pretty much an extra £100 a month that buying with finance will cost you over the best leases you can find. 8p a mile excess charge is very reasonable if you need to do a bigger annual mileage.
With you not needing finance at all, that's £27k you could have sitting in the bank/ISA, earning interest for you if you leased on the same average monthly cost as an interest-free £27k loan.
If you were still wanting to buy outright and have nothing to part-ex you could just go straight to a broker and pay £27k.
