I think the GBP 35 you paid is only for the first year. There after they will take about double up - so about GBP 70 automatically. And they tempt you with the 1/2 price offer and hope you forget about it and then kick yourself in the backside when they "auto debit" your card with the full price next year - and probably without warning if I'm to predict future behavior based on past behavior.
I to think Parallels has become a parasite company - paying > 50% every year for "upgrades" is crazy. Especially since you can probably pick it up for the same price in a bundle at some point.
From the purchase page:
This is a subscription product with a fee of £34.99 per year. You may cancel your subscription at any time.
So I do not expect the subscription to increase to ~£70 next year