The only way to find out is to phone up and ask. Forum staff aren't allowed to offer sales advice or make package changes.
Call up, select the relevant options for package upgrades. if you're currently in a fixed term contract, any existing discount will vanish, and you'll be started on a new 18 month minimum term, with the usual heavy exit penalties. If you're out of a fixed term, then you can try your luck negotiating a discount to stay on your existing speed, and when you've got the best you think you'll ring out of them for that, ask "and how much if I go to 1 Gbps?"
It is possible to negotiate a discount straight up on 1 Gbps, but it's difficult because you've not got much leverage. If there's a moderately near date for Openreach or another FTTP provider availability that might help.
I ended up having to pay a bit more unfortunately as my previous package had a loyalty discount on...
Plus a condition of it was I took a £25 a month o2 sim... is this definitely a requirement? I don't even need it unfortunately.
the £25 sim is a VOLT deal - looking at the website thats certainly is available but so is 1gig BB on its own - thats to new customers so any price shown will be for new users but i dont think the deals are any different for existing customers - just the price which you need to negotiate
imo they have mis sold you the O2 sim especially as you do not want it
that said what the price of BB only would be is the thing - they are pushing the volt deal so they may do better deals
but if you stay with the volt deal i would have thought you should have signed up for the 500 BB speed and then doubled it for free
there has been confusion in what the deal actually is - who did you speak to - if you did it via chat then thats offshore and they are not the best - you need to speak to UK retentions - so try this
dial 150 or 0345 454 1111 and follow the prompts to - 'thinking of leaving us' - dont worry about that they do much more - open 8am -10pm 7 days a week - should take you to a UK call centre
if you get offshore then hang up and try again or say you want to give 30 days notice - that should get you transferred to the UK - once there just tell them what you [actually] want to discuss