Btw that £35pm 'exclusive deal' is not that exclusive. It's the standard price on the website for new/existing customers. What i'd do is ring up and say you're leaving and ask them to put you through to retentions to get your pac code. Make sure you have the three website up on a laptop nearby as well as the third party sites with the deals. Just tell them you've found better deals on these sites and were wondering if they could price match. They will say no because they're third party and make up some crap about how the cash back is false or how the site doesn't exist.
Just ask them if they can do anything similar and keep pushing them until they offer you something good. Every offer they give you, check it against the three site just to check if it really is an exclusive deal and not a standard one. Keep telling them that if they don't offer anything better than the standard deals you'll get your pac and leave/.
It might take a few calls back and forth between you and three before they offer a good deal or before you decide to leave. BTW make sure you're in your last 38 days of the contract before thiking about upgrading as they only offer good upgrade deals during that period.
FYI- Three have kind of stopped with the discounts on high end handsets recently. Anyone on Three here will probably remember how recently they'd give out pretty good retentions deals over the phone, they seem to have stopped since the new year.