That's the one I have, it works very well for Three's 3G which we used for years - reduces latency and improves speed in both directions. Basically, I've played around with this kit for years, you've gone from nothing to what I have, in about ten days.
Demonstrably I have had and continue to have surprisingly good speeds, to answer your question - fluctuating speeds are a bit of a feature of mobile broadband in most places, we're just lucky here. The speed will depend on the number of users using it, how far away it is/what your signal strength is.
If the speeds wander spectacularly, this might indicate the activities of other users, alternatively, if you have two cells near you at roughly equal distances, your router may be flipping from one to another (the drop outs?) and one is more perrformant than the other. That's what the directional antenna does - effectively forces it to latch to a specific cell; the one it points at.
When you get your antenna - it's surprisingly heavy - try poking it out of the window upstairs and see what result you get to start with. Plug it into the left hand socket on the back as viewed from the back. Log into the router and check the bottom of the front page "Antenna Status". Does it go to "Auto" by itself? If not, you can force it to use it in the settings. You may need to do that while you experiment, with your router stats (even mobile broadband users can have stats, it's not just for ADSL!) on the screen.
You do now need to find out where your local transmitter(s) is/are so a few minutes with EE's coverage map, perhaps Google Maps and your own local knowledge should get you there.