I suspect that some of your concerns, @burndelight are more to do with, if/when it all comes crashing to a terrible halt - you and your husband will lose your friends. That your long-standing friendship will be uncomfortable, awkward, and irretrievably broken, because an engagement or a marriage between your son and their daughter's ended.
Actually, I get that. The kids dating...? Eh, it's nothing serious, is it...? Just a bit of heart-warming fun! Now that their 'A'-levels are over, and they're legally adults (although still very young, incredibly sheltered, and about to discover that the real world of uni... is nothing like they've actually imagined it to be!), it's natural that they're making these plans. They're a safety net for one another. And that is very sweet, but probably unrealistic in the long-term.
Sometimes these things do work out (my mother was 19 and my father 22 when they met... and they were husband and wife 6 weeks to the day later! 53 years on, they're still together) - but very often... they don't. My son's father and I were 11 when we first met, dated on/off between 14 and 20, 21 when we got together "properly", 28 when our son was born, and 33 when we split because he cheated. Apparently (according to his mother) I "held him back".
If anything, it was the other way around. He travelled, but I went to university. He went out with his mates, I stayed home with the children (I have a daughter from a relationship that went sour when I was 18/19). He was free... I had all the responsibilities. One of my oldest friends met a boy at 18, married him at 22 and was divorced by 24 because she felt stifled by him. Another friend met her husband at 4, we're all now 46/47 and they've been (very happily) married for 25 years with two lovely daughters.
It's swings and roundabouts, OP, and unfortunately you don't know which way this is going to blow. You don't want your son hurt, you don't want to lose your friendship with the girl's parents. All you can do, is trust that your son knows what he's doing, advise for a long engagement, and support them in this. University and the different sets of friends they'll make may well tear them apart - but they're clinging to the familiarity of one another, because of how anxious this first step towards leaving home and growing up is making them.
As long as the holiday isn't to Vegas, it'll work itself out gradually, I'd imagine.
And no; it's not easy. My son's 18 and the thought of him tying himself to one person for the rest of his life at this age is... downright terrifying. I get it, OP as do most of the other posters. 