Sycamore is a very complicated issue!
The main points though are;
They aren't native - but a good bet in some areas for the future in terms of both climate change and the many new (i.e. new hybrid origin) phytophthoras we now have in Britain. Sycamore are relatively resistant to these diseases.
They support relatively few species of invertebrates, but they support very high numbers of the ones that do feed on them. i.e. They support a low level of biodiversity directly, but a very high biomass of certain aphids - which are great for may birds - tits etc and dormice. The variety of insects in a sycamore dominated wood is typically much lower than oak/ash dominated, but the total weight of insects is typically much higher.
The pH of their bark allows them to support similar lichens to elm (which for obvious reasons aren't as common as they once were), and as such are a good alternative host for these species until the cycle of Dutch elm disease allow elm to become prolific once more.
A new planting containing lots of sycamore is unlikely to ever be dominated by sycamore in the long term, as grey squirrel will preferentially hammer them once they are 10-20 years old! Some people trying to grow beech include sycamore in the planting as a sacrifical species in the vain hope that the grey squirrels will leave the beech alone and just damage their preferred sycamore - I've heard varying degrees of success for this.
In many ways it's your call about what your motivation and aims are! I would include some, but certainly not more than 15% myself.
Removing non-native sycamores from an ancient woodland site is a very different story. Ancient woodland is irreplacable. Planting fields with trees won't replace an ancient wood. The soils have been irreversibly changed by agriculture, and it will never support the fungi and flora of an ancient wood. It's certainly worth taking sycamore out of ancient woodland sites, but when planting a new wood then depending on the aims it can be a valuable component.
If I was planting a wood in lowland england with conservation as a priority to the two species that would dominate my initial planting mix (i.e. not necessarily what would be left to dominate the mature woodland in the longer term) would be small leafed lime and sweet chestnut.
Small leaf lime (Tilia cordata) was the dominant broadleaf tree over most of lowland england until humans started favouring oak - because it's so much more useful to humans - acorns for pigs and very durable wood.
Sweet chestnut. Not native but it grows very quickly, and so in a new woodland gives you large diameter trees very quickly, which you can ringbark to create dead standing wood when other species finally reach larger diameters. In a new woodland planted from scratch the thing missing is deadwood - either on the ground or standing. Deadwood is what most woodland specialist invertebrates need. Sweetchestnut can provide this quicker than any other durable species - i.e. fast growing willow/poplar don't give the best sort of heart wood to create deadwood from. Also if timber isn't a priority sweetchestnut recover from squirrel damage better than most species - they callous/heal wounds particularly fast, a damaged tree will never give you quality timber, but it is less likely to have the top blown out by the wind before it's healed like birch etc. would.
Don't underestimate what squirrels will do to your newly planted wood just when it's reaching a satisfying stage!!! Remember the woods you see around you grew up before grey squirrels were here - and they've only been at these current crazy numbers for a few decades which is nothing in the life of our woods - sadly they are now pretty much the keystone species...
Good luck!
