I think the VDSL one in the UK still the STD firmware
Try the vector 1 & the *all* file (you will see it in the folder once downloaded) from main Taiwan & the link above, is larger file from the other 2 which are in the UK download page
The only difference it may be the modem code but other user tried is still good for the UK lines
Have nothing to lose by trying it

Actually, I've just gone back to 3.7.8 and it is labelled "Hardware Acceleration" not just Acceleration and it is between the NAT and the Firewall menu items.

yea this is the one ... however using the RST file extension after a major firmware update is better
The *rst* firmware will set everything back to factory defaults but is better for a clean instal avoiding conflicts & bugs from previous settings & new added features
I know most users prefer not to use the *rst* file because they have so many settings in their draytek units but at least (still not the best option because can cause issue with the latest features added) can use a backup

Well, I did a full reset using the hardware button followed by using the 3.7.8 RST firmware again and this time enabling the Hardware Acceleration option (Auto) has not killed SSH. However, I am now using WAN2 connected to the OpenReach VDSL modem rather than using the router's VDSL modem as I'm being DLMed like mad after all the reboots I've done.

I wish I knew exactly what this Hardware Acceleration option did though. It seems to be related to prioritising downloads in some way, but more than that I can only guess at.

It certainly uses a lot of CPU power when downloads are going on when it is enabled.

For those who don't have it an are interested, the options are:
Disabled
Auto
Manual

In Auto mode there is nothing you can adjust (as you might expect).
In Manual mode you can select TCP and/or UDP then you choose one of three further options:
1) Accelerate heaviest traffic sessions
2) Apply the Class Rule in Quality of Service ("Class Rule" is a hyperlink to the Bandwidth Management >> QoS page)
3) Specific Hosts: (you can specify Destination port start, Destination port end, private IP [or click on the Choose PC button to select from a device on your network])

There is a note at the bottom that says: Bandwidth Management will not work if Hardware Acceleration is enabled. Currently, Hardware Acceleration is supported only for download sessions.