Hello, thank you for all the replies! Going with the unanimous recommendation for the IB widebody.

I was originally booked on the BA7054/IB3162 MAD-LHR 0740-0915, operated by IB A350, connecting to 1240 BA169 to PVG in WTP, and had no choice of BA as the first BA only arrives in LHR at 1215.
BA's MMB through-link to IB allowed me to select the 11A PE seat for free, so an all PE (hard product) trip.
I was happy about this but started to get worried about the MAD-LHR being late and/or documents issue at MAD check-in so thought about taking the MAD-LHR the night before.
Also, 0740 MAD flight would have needed a 30eur morning taxi, but for an afternoon flight I can take the free Madrid airport train; the T5 Premiere Inn is 58eur so it's just 28eur 'non-fare difference' for a much less stressful journey.

re: IB LHR T3/T5; lamented the change back to T5 though it does make make connecting to T5 BA168/169 easier. Will do a tour of all 3 GCs....

re: Travel documents, my 'niche' RHP is not listed on EF's Timatic...thankfully trip's not till 2024 (yep over thinking and far in advance )

re: BA/T5 reliability, hope it improves soon.