It’s very very hard and nearly impossible over the phone.
On live chat I managed to get them to offer it and then recorded the chat and emailed it to the advisor I spoke to on the phone. The advisor on the phone said they can’t add discounts to an annual upgrade but once I emailed her the chat she spoke to her manager and they said they will manually deduct £10 off my bill each month.
on live chat you need to be really nice to them and say you will give them good feedback, then say “please can you give me a loyalty discount” they might say no but keep trying and if they insist no then say “I spoke to someone yesterday and they offered me a 10% loyalty discount so please speak to your manager and check this again” - both times I’ve been on live chat I’ve successfully had this 10% offered, you just need to keep insisting.