The general rule of thumb is that an Annual Pass is only a good idea if you will break even or better for park entry alone. The additional perks are nice, but the only one that makes a huge difference is the room discount, and those are not guaranteed. In order to take advantage of this greatest discount, you would have to stay on property, so you won't have to pay for parking anyways
I'm guessing you live in the UK, since you mentioned a 21-day ticket. This option isn't available to anyone outside the UK.
I suppose if you are staying off property, then you will be able to make up for the difference in parking fees.
I estimate the fee to be about 10 GBP/day, so if you park at the parks every day during your trip, (21*10= 210) you'll make up the difference between 2-21 day passes and 2-Annual Passes.
Something else you might consider is that most of the AP discounts only require one Annual Pass in a party.
Parking is per car, so if you're all riding together you only need on AP
Discounts on Water Park, DisneyQuest and hard ticket events require 1 Annual Pass for 4 people.
Mini Golf - 50% off for passholder and 3 guests
World Showcase lunch discounts - passholder & 3 guests
Some of the discounts are only valid for the passholder, so you should read through all the available discounts before making your decision. For the most part, with a difference of 100 GBP/person, even with the few things (i.e tours and experiences) that require passes for all participants you'll still probably come out on top with just one Annual Pass per 4 people in the group.
http://www.wdwinfo.com/discounts/discounts-ap.htm#general