There is no way for us to answer a question like this. You need to ask 2 or 3 garden designers to give you clear quotes.

I'm also not sure what you mean by "feature garden", and even if I did, not enough info in terms of size, current and required planting and landscaping.

I live in Lancashire as well and we are developing a garden out of a derelict woodland plot.Our costs have been as follows:
hard landscaping (terraces, steps etc) £4K (labour= £100/day/person: materials = £1K but we are reusing a lot of original stone on site, so to buy it in would treble that)
fencing (timber and wrought iron) = £6K (but it is a large plot)
top soil = £30/ton delivered - we had 3 tons(need more)
gravel = £30/ton - 3 tons delivered
fence posts for raised bed edges - 230 at £2 each
Paving - original flags are costing £50/sq metre - if you can get them - our costs = £1200+
Plants - probably cost £1K to date, but we are still buying!
our own labour = nil cost but probably totalling hundreds of hours!
We have our own digger (£5K off ebay) and concrete mixer: dread to think what it would have cost to have hired them for the length of time we've used ours.
So even with a DIY build (and some sub contracting) and keeping costs down by shopping around and using reclaimed materials where possible we have probably spent £19K. over the last 2 years and it's still not finished.
If you are wanting it designed (and I did make enquiries) most companies will only do design and build and prices can start at £10/25K for the design alone and obviously the sky is the limit if it is a big plot and you have a large bank account (NB 2 garden designs on TV recently cost £125K and £250K to design and build but they were fabulous and I could see where the money had gone based on my own experience as stated above).
f you know what you want + size of plot and task you could go to the Tatton show in July and speak to some of the garden design exhibitors and get some rough ideas of price. Some of the larger garden centres near Preston and Leigh have in-house designers - you could ask to look at their portfolios.
Hope my experience of the costs of trying to create a garden are useful to you.