Probably Timesnatch (1994) by Robert Swindells.
Calling the ship a "doughnut:"
Kizzy gazed at her mother's machine — a machine which could vanish and reappear like something in a magic show. It reminded her of an enormous doughnut. In fact, she'd christened it 'the Doughnut' without telling the other two. 'It's absolutely fantastic, Mum,' she breathed. 'What will you call it?'
Selling information to a journalist:
Bilbo shook his head. The journalist sighed, produced a fat wallet and counted out some notes which he folded and passed across the table. 'There you go — there's fifty E there. That's for the information you've given me, and for your discretion. Know what discretion means, do you?'
Dealing with Nazis:
'They're Nazis,' snarled Harper. 'A bunch of Nazi thugs who can't muster a decent brain between them. Take your hand off my wheel, please — we're going.'
Disposing of it in a volcano in the past:
Harper laughed. 'There are no bits, darling. I didn't destroy it in the lab. I'm not that daft. I piled all my papers into it and sent the whole caboodle into the past — two thousand years into the past — and to make absolutely sure, I dumped it in the mouth of an active volcano.' She smiled. 'So — those boys back there are searching in vain. There's nothing left but some shorted-out circuitry which might have been used for anything.'
A well-known volcano:
'Which volcano would you have chosen?' pressed Harper. 'Which name comes into your mind whenever you hear the word volcano?'
'Vesuvius,' murmured Kizzy.
The book is available to borrow at the Internet Archive.