A few things to consider:

- Any telephone extensions or even disused wiring going to elsewhere in the house will have a negative impact on your broadband speed. If possible, connect only the DECT phone base station to the socket with a single ADSL filter and have extension sockets (pretty much obsolete these days) disconnected.  Use the master Openreach socket or have it moved, if possible.

- There is a finite potential speed you can achieve depending on how far you are from the cabinet. Use this chart (ADSL) or this one (FTTC) to determine what you can reasonably expect as a maximum speed.

- Whatever speed your router can achieve is not necessarily going to be your maximum speed over Wi-Fi.  That depends on whether you are on wireless-g, wireless-n or the latest wireless-ac and how far you are from the Access Point, whether there are any thick walls, steel RSJs etc.

One suggestion I would make, never use your provider's own speed test.  I'm obviously not suggesting that it'll be weighted in the company's favour, but...

I've used this one for years, and believe it to be accurate.  It's also quite a useful site in general for anyone with a passing interest in broadband etc.

The first thing I'd do is this... If possible close the children's tablets, X-Boxes etc, the wife's laptop, your spaniel's iPad, everything in fact, except your 'main' device.  Said main device should be connected to the router directly, via ethernet.  Run your speed test.  Run the same test at different times of the day and night if possible.  This should give you an accurate average reading.

Connect your main computer as per normal (back to wifi if your main computer is not normally connected via ethernet) and run the same test(s).  You can now see the degree to which your wifi is degrading your actual connection.

Tell, the wife, kids, dogs, lodgers, the bloke next door who's stealing your wi-fi, etc. they can reconnect to the outside world and then run your speed test yet again.  You will then be able to see to what extent every other bugger in the house is strangling your total bandwidth.

The chart that Martin linked to is another useful tool to get a rough idea of the speed degradation you can expect over distance.  This, of course, will vary if you have FTTC (Fibre-To-The-Cabinet) or FTTP (Fibre-to-the-Premises).

From the *very* not technical.....

Turn everything off for a few minutes, maybe longer. Perhaps all off when you go out.
One item only back on the router and see what happens.
Bring them back on one at a time.

We have 17mb and that works fine.
Average of one ipad and one laptop connected at once, could also have one iphone, 2 x Amazon firesticks.
If we have visitors add up to 3 iphones and 3 ipads.
All seems to work fne.

The non technical didn't know that

That's only going to work if you get close to the router. Better still, do a test from the router on an ethernet connection to get your baseline internet speed. Then experiment with wi-fi.

Do you not have a laptop you could plug in for testing?

The more I find out about life and technology the more these things fail to surprise me.

Seems just about anything is possible

Addressing the question in general.

How recent?
If you hadn't noticed, the schools have been closed recently because of a moving holiday period revolving around Easter.
Which means a lot more kids at home playing online (heaven forbid that any one of the treasures scratches their knee by playing outdoors, which is what my generation did), or watching Netflix, Pornhub, etc etc ....

You mentioned home, and you mentioned Sky.
That should be a clue.
Sky do not do business broadband.
Domestic broadband services have a much higher contention ratio than business users have. You could be having a ratio of 50:1.
Business users pay substantially more for their services, and because they pay more they demand greater dependability and higher speeds, and they get them.
Their contention ratio varies from 1:1 to 20:1 -nit all depends on how much they spend.

Then the killer blow is this - the faster your service is, the more sensitive it is to little things such as RFI, and line noise doesn't help either.
The RFI issue can be alleviated by the use of an up to date telephone socket, which is what has happened with BT provided services.
The up to date sockets have filters that improve connections by filtering off the worst aspects and effects of RFI on the circuit.
Another thing to check is that your BB filter is a VDSL one.