There has been a lot of useful information in this thread and I have just done my first of four runs. I thought I would take a different slant and try and using a Mobile Boarding Pass (MBP) as much as possible (mainly because I hate carrying bits of paper). Hopefully some find the information below helpful, even just in terms of Online Check-In (OLCI)

Background

• All four tickets were booked direct with UL using an Amex card. I emailed the required documentation for the Mandatory Plastic Check and received an email back stating "Passenger does not need to travel with credit card".
• OLCI was always done on a desktop browser after T-48, never using Srilankan Airline's App, nor checking in via a browser on a mobile phone. In all cases this resulted in options for boarding passes being emailed to me (PDF and MBP).
• As well as emailing myself an AppleWallet boarding pass, I also carried a printed copy with me just in case.
• I intentionally ignored previous comments in this thread about mobile vs. paper boarding passes on the basis that if the UL website allowed it, a MBP should suffice.
• Naturally, I went Hand Baggage Only, so never went to a check-in desk.

LGW - DXB

• I flew EK as I am Platinum with QF so have benefits here and the flight times were better than BA to DXB or AUH. I mention this as the transfer from T3 to T1 is via a bus every 30 minutes.

DXB - CMB

• No issue with security at T1.
• The Marhaba Lounge accepted my MBP without question and I was escorted to the premium section of the lounge.
• I got the beep and red light when my MBP was scanned on boarding, presumably because I had not used the check-in desk. A few details were entered from my passport and I was on my way.

CMB - CGK

• I wasn't sure where the lounge was so asked the Transit desk and showed my MBP. This was a fail as he printed off boarding passes, plus a lounge invite. And looked distinctly unimpressed with my MBP asking "What's this?", although I gather this is their general demeanour.
• The lounge welcomed me and retained the paper lounge invite (and offered me a free foot massage) and I showered my airport printed boarding pass.
• The airport print boarding pass was used on boarding the plane, only because it was in my hand after the security shambles.

CGK - CMB

• From what I saw everyone else had paper boarding passes for all airlines, however both security and immigration were fine with my MBP.
• The Plaza Premium Lounge accepted my MBP without question.
• At the gate my MBP was not accepted and I was issued with a printed boarding pass, also for my connecting flight CMB - AUH. Of note is they board the rear of the plane first but hold them in the aerobridge. Then Business Class walk past them to board.

CMB - AUH

• No issue with lounge access now that I knew where it was. MBP was used to print off the lounge invite that they just stored.
• No issue with security or boarding with MBP. Remote stand, which was painful!
• No issue with boarding.

AUH - CMB

• This was B2B. Security was empty and no one showed any interest in my boarding pass, including there being anyone at the top of the escalator. Didn’t even need to take out laptop / liquids. Simplest B2B you could ask for and I was never 100m from the UL plane (same plane and crew back to CMB).
• The Al Dhabi lounge had trouble scanning the MBP barcode but it wasn’t an issue being granted access.
• Usual issue at the gate where “Not permitted to board” flashed up. Entered a few details from my passport and then told me I was good to go.

CMB - CGK

• Again, not issue with lounge access, even managed a free foot massage.
• Security wasn’t convinced it was a boarding pass but let me through the scrum. Honestly CMB security at the gate is horrendous and they seem to board two gates at once!
• MBP scanned fine but they wrote down my seat details and placed it on a board.

CGK Immigration seem to insist you are there for business, despite saying “no” multiple times. I was asked for proof of onward travel and used my paper boarding pass as scrolling through a very long list of Srilankan Airlines MBPs may have caused more questions to be asked.

Edit

• If you are using OLCI and HBO, meaning you go direct to the gate, then make sure you have a return ticket to your home country as this will save some time and hassle with the gate agent who does not want to board someone at risk of not having an onward ticket / valid visa. They will check their system to see if you are eligible for a visa on arrival in AUH/DXB, as I am being a UK citizen.