Hi DevonWoodlands, welcome to the forum. And a thank you for the 'shout out' from @harvey b davison.
Firstly, to put things into perspective, the Honda HF1211 which has given you such great service was actually produced by Stiga for Honda and fitted with a Honda engine. I have just this evening delivered a new Stiga version, the 2072H.
The fact that yours has done so much work is testament to the quality of Stiga machines and as it is one of the cheapest at £1799, and one of the smallest, you can be assured that bigger and more expensive Stiga models will be better still.
You are correct that Stiga do not offer the larger and heavier duty Diesel machines such as the Iseki and Kubota, but based on the service your Stiga built Honda has given you something like the Italian built 92cm Stiga Estate 5092HW at £2799 will give you sterling service and with the 290litre collector will go further between emptying than the 170litre box on your Honda.
The 92 cm machine has a V belt drive to the blades unlike the 42" and 48" versions which have a timed deck, and with your tree roots I would keep clear of timed decks as you will inevitably have timing belt failure. So if you would prefer to only spend £2799 one of these would do the job fine.
Stiga Estate 5092 HW
But to be honest I would not contemplate cutting the amount you do with a collection mower. I would go for the super high quality Stiga Park 740PWX with 110 Combi Pro deck.
Stiga Park 740 PWX
I know its a mulcher, but its a joy to drive and excellent around trees and paths. It leaves an excellent mulch finish although it has to be said that the best finish will be had on more frequent cuts so you may want to do your fine lawns areas a little more regularly in the fastest growing season. Your wooded areas will grow slowly and so will be fine fortnightly.
I am confident that with the increase in size and power, and without the need to stop and empty, you could cut your mowing time to about 3 hrs a fortnight allowing you to maybe cut a little more often in high season.
Please feel free to give me a call if you wish to discuss further