Thanks for everyone's input.
I just thought I'd post an update.
I took the plunge and purchased the TP-Link Archer MR200
4G router, along with an EE Data SIM
with 6Gb of data (lasting 90 days) for testing purposes. I did also get the Poynting A-XPOL-0001 external antenna
as well, in case it was needed.
I was a bit nervous, as I can't get a 4G signal via EE at all on my phone inside the house (and there's only one location so far in our field where I get a 4G signal most of the time). I haven't yet tried with other networks (Vodafone may be worth a go here too).
Anyway - I had success in the window of 1 of the upstairs bedrooms - after much trial & error in positioning the modem (I had to put it on a small pile of boxes to gain height - now looking for some kind of adjustable windowsill stand/shelf), and also after *forcing* it into 4G mode (otherwise it would always pick up a 2G or 3G signal), I eventually can get a consistent 1 bar (out of 4 - i.e. 25%) 4G LTE signal!
So, comparing my PlusNet ADSL - these are typical speed test results:
SpeedTest.net - PlusNet ADSL (on my 20CN Market A exchange)
ThinkBroadBand - PlusNet ADSL (on my 20CN Market A exchange)
(Speed has dropped back to it's usual 2.8Mbps down, 0.37 Mbps up - I can't complain too much, others nearby are worse).
My 4G router with EE SIM gives these (it is variable, and this is one of the better test results):
SpeedTest.net - EE Mobile from 4G router
ThinkBroadband - EE Mobile from 4G router
It does vary from around 10 to 35 Mbps downstream, & 1 Mbps to 15 Mbps upstream - according to router position, signal strength, weather conditions, etc.
But it's huge improvement! I've been able to extend the range of the router via TP-Link powerline adapters (with one also acting as a WiFi extender).
Also - the Poynting Antenna so far hasn't made a difference, in fact I get a stronger signal via the router's own antennae! However, I may need to use it later, as we have a row of Beech + Ash trees nearby, and come the summer when in full leaf, these may adversely affect the 4G signal, such that I may need to put the external antenna on a home-made mast/pole beyond the trees (or on external wall of house) to improve things.
However - the 4G does provide a huge speed boost over my ADSL link.
It's just a shame about the costly data caps!! At least for mobile data contracts, it usually less than £1 per Gigabyte of data (and sometimes special offers mean it's closer to 50p per Gb - I've seen past deals of around £15 per month for 32Gb), so maybe things are improving.
I think 4G would be a viable "booster" for my slow-ish ADSL, and also provide an excellent backup in case the landline has any problems - but is still not a substitute for improved fixed-line NGA broadband. A good FWA link *could* be a substitute, so may investigate that option further down the line (but I'm a bit dubious about LOS [Line Of Sight] issues, due to trees etc).
But, at least it shows that 3G/4G (and upcoming 5G) could help to improve the situation in slower broadband areas.
Thanks again for all the input.