wonder why they werent intalled by Boeing at delivery.
At the time, they weren't certain they were going to need it. The HKT route wasn't confirmed, and route planning can be a fragile thing. It's a lot of weight to carry if you're not using it, they reduce your cabin and galley space too.
Either they thought they could wing it without them, or cost.
You can't "wing it" with flight time limitations.
Cost isn't a reason either. It's a Boeing made kit and you pay for it whether you fit it at Stansted or Seattle. Shipping the kits to the UK and leasing a hangar is hardly going to be a low budget option.