I used to use Northolt quite often - mind you, this was 4 years ago. Staff were never anything but friendly and helpful.
Then it was exactly as 'oldfella' described, followed by a quick walk to Ruislip Gardens, then the Central Line into town.
But during the Chancery Lane aftermath and the closure of the Centrl Line, I went to Uxbridge instead.
Staff were OK, I just told them I was going to a meeting in town and wished to park in a secure area. Which was true, hence the jacket and tie. But parking wasn't as straightforward, it was a longer walk to the station and the journey to the City took ages in a noisy underground train.....
But from British West Oxfordshire, the Didcot/Paddington option is a lot easier - no battling with the M40 or Oxford ring road!
Perhaps an RAFBF donation (say £1 per day) to park at Northolt wouldn't go amiss?