The easiest way is probably Iberia SDQ > Madrid and then Madrid > London, the connections work very well (6am arrive Madrid and onward flight to London at around 9am). Transit via the US is always a nightmare. Iberia is definitely flying and as BA's sister company, BA will happily book you onto Iberia. Be aware that when you arrive in the UK you will need to self-isolate for 14 days, and they do check. A friend of mine just came in and they have been calling her every day and tracking her location via her mobile phone.