Thanks again for all the advice, which I've now discussed with my friends.
There's one bit of info that I was not aware of which makes Peter's
suggestion regarding parking near Purley and Mizter's regarding Anytime Day
Travelcards the most relevant (and which explains further my friends' wish to
drive part-way). Apparently, they want to shop at Wing Yip supermarket after
their visa trip to the Embassy! So all settled now, thanks all. Will.