I'm taking MrsSquawk to Reykjavik using a RFS for the weekend in November returning on the BA801 at 15:30. How late can I realistically leave it to clear security at KEF before the proposed 15:00 boarding time? Ideally I want to squeeze in a trip to the Blue Lagoon on the way back to KEF; there's a bus back from here to Keflavik at 14:00 (20 min transit) - but will this leave enough time?

Thanks for any useful advice.