So MS run deals to upgrade your XBox Live Gold to GPU for £1, and when you do that it upgrades all your existing Gold time to GPU. So the cunning trick is to buy 3 years worth of Gold (assuming you have none currently - 3 years is the max you can have) for £120 using CDKeys or the like, add it to you account being careful to refuse the extra month they offer for turning on auto renew (this would stop you adding the final year as it would take you over the 3 year limit), then upgrade to GPU for £1 and tada! 3 Years of GPU for under £120.
NB: If you add Gold when you already have an active GPU sub then it gets converted at a much lower rate than 1 month == 1 month. You can still reuse this “trick” when your time is up but you need to let it expire. MS appear to be aware of this but accept it as a promotion that gets subs in.