It's not a pruned node, so it looks as if will need 800+Gb of storage
Isn't a pruned node much easier for your purpose? You'll only need about 15 GB of disk space, and it still verifies every transaction you make. The only thing it can't do, is sync older wallets or load older keys.
I'm going to have to move the Bitcoin directory onto an external USB-C drive.
I don't really like that: it's so easy to accidentally disconnect it, which will corrupt your data. I've had disks disconnect with the slightest touch to the cable.
I'm not bothered about anonymity
If that's the case: what's stopping you from just using Electrum? It's so much easier and faster for your scenario.
- maintaining a backup, and I plan to do this with a second drive containing the blockchain and wallet details.
Do you need to backup the entire blockchain? Just your wallet is enough to redownload everything when needed. I prefer a few MB over 800 GB

- provide an accessible record for the inland revenue so that I can minimise my tax liabilitty..
I'm not familiar with your tax system so I'm not sure what you need, but have you seen Blockchair.com's "PDF" feature? Search an address, click PDF, and it shows you everything there is to know about that address in a printable format.
- Will I have any problems if I move a drive from one machine to another, and I ensure that core has the correct entry for the external drive?
It should be okay. With Bitcoin Core, you can even run them both at the same time. But I wouldn't recommend it: every copy of your wallet on each different system adds to the risk of someone compromising your wallet.
- What happens if I make a payment on drive A, and then use drive B on another computer? Obviously receipts will update as the blockchain synchronises, but will payments do the same, or will I have to copy the wallet files? I suspect that I will have to do this.
Transactions (incoming and outgoing) update while syncing, but labels are missing. Electrum has a LabelSync plugin, but I've never used it.