Yes, it is definitely possible to insure cash theft within a travel insurance policy. I would recommend however that you minimize the cash you have on you, and document clearly your cash flow through receipts from banks, money exchanges and ATMs to prove that you had the cash on you.
Regarding the express kidnapping, there are insurance policies existing against this. Since those things happen especially in certain countries (Mexico, Brazil for example), it might be wise to insure against these if you have to go to these countries. I would not think that it is worthwhile getting such special insurance for other countries such as in SE Asia where this does not exist.
You might have to look around for companies who insure against these specific issues, since UK citizens might be a much smaller market for express kidnapping insurance than for example US citizens living near the Mexican border. So to find a company that offers this in the UK might be difficult. It would be wrong however to say that such insurance does not exist. I would also doubt that this is automatically covered by your bank.