Nothing conclusive, I'm afraid, but a couple of things that occurred to me/ I came across:
- Given the announcement gives the mother's name as 'Sarah Sherlock' (the latter presumably being her surname), isn't the implication that the couple are not married?
- Twitter seems to indicate Penny Johnstone (former BBC presenter [this a strongly corroborative point, as a lot of references to their marriage mention her BBC career], now in equine-facilitated learning) as living in Cumbria; things she's retweeted clearly identify her son Percy, this being also the name of the son of David, Lord Johnstone [etc]; of course, this is only of any use if we know David, Lord Johnstone [etc] not to be himself resident in Cumbria- if we KNOW that he's still at Dumfries, the likelihood increases of his being the 'David Johnstone' mentioned in the announcement/ separated (or divorced) from his wife.
Sorry, nothing at all definitive, but the above thoughts might (albeit unlikely) help someone else out!
EDIT: A-ha; Penny Johnstone's LinkedIN page gives us: 'Hospitality & Development, Raehills Estate, Mar 2005- Jul 2012, St Ann's, Lockerbie, Organiser of high-end sporting hospitality' (the end of this 'position' presumably representing also the end of the marriage?) then shows her to have been a journalist for BBC Radio Cumbria in 2015, and an equine facilitated learning practitioner in Cumbria since 2016. The David Johnstone in the document linked above gives the address as 'The Estates Office, St Ann's, Lockerbie, Dumfriesshire'. Raehills is, based on an internet search, owned by the Hope Johnstone family. So from this evidence I would tentatively conclude- IF David, Lord Johnstone is pretty certainly still in Dumfriess- their marriage has ended and the announcement does indeed, as you thought, Richard and Brooke, refer to David, Lord Johnstone.