As well as calling our own county council, I also emailed a few other locally (for some reason, we live in an area with an embarrassment of riches in terms of councils) and a couple of them have been back in touch. I have ascertained that they would definitely be interested in taking on prospective adopters of a baby under 1, and that we don't really need to start applying now (though perhaps in 6 months time). They also sounded very positive in particular about our interest in continuing to meet birth family.
I literally just spoke to a social worker who is finding a family for a 5 month old baby. Literally. And in US terms, this is a legally free baby. As she and I both said, adoption in the UK has changed a LOT in the last 3 or 4 years.
I have not, however, told them our ages. One further council is calling back next week and I'll see what they say, or ask. I am assuming (perhaps wrongly) that if they wanted to know and it was an issue, they'd be asking. Or perhaps they assume that, with an 18 month old, we aren't going to be any more than 2 years older than "the right age to adopt a newborn", in 6 months time. Which isn't really rocket science. So I'm not mentioning it till I'm asked.
So we will see. These aren't routes that are definitely open to us, but they are possibilities. From where I'm standing, I look at Baby Spouse's tiny baby pictures and am sad that we might not have that again. But then I think about how annoying our county council are being and wonder if we'd get over that if we wanted to go with concurrency.