I would keep the speakers you have and upgrade the amp to something really nice. If you replace your speakers, you'll be unhappy with the amp, then you'll need to upgrade it and you'll have to choose one that's good with the speakers you already bought. Not ideal. You already have the right level of speakers for your amp (maybe even better) and going for better speakers with the HK won't necessarily sound better.
If you upgrade both, you'll split your budget. You could get a nice amp/speaker combo for that price, better than what you've got, but not THAT much better, and now you've blown £1800. And next time you upgrade, to do better, you'll need to spend £5000 to improve things.
If you go buy a really sweet amp with all or most of your budget, it will make your system, with the existing speakers, sound phenominal. If you are a student then I'm guessing your listening room is fairly modestly-sized. Those MA speakers will reveal the difference between a Harmon Kardon and, say, a Leema Pulse or Naim XS without any problem! Try it - take your speakers to a shop and ask to hear them with a high-end amp. You will be amazed.
Then you've got a real amp that you won't need to replace for a long time. You can take your time searching for speakers that match it. Maybe find something used, or just hang on and save up until you've got a bigger (or just different) listening space and can buy something that matches well. This strategy gives you a huge upgrade now, and then later - since you already have the amp - you could have a truly high-end system just by buying nice speakers (far less than £5000) that fit your room.
One other suggestion: At that budget, it's crazy to buy a new amp. It's great that you have a good relationship with a hifi shop and can bargain a little, but I bought my Pathos Logos, used, in perfect condition, for less than your budget. That's a nearly £5000 amp! For the first three weeks I had the amp, I listened to it on a pair of KEF Q300s - in the same price neighborhood as your speakers. It sounded stupid good. Now my amp wouldn't work for you as you need a phono stage, but still: if you are going to spend £1800 on a chunk of solid state electronics, the difference between buying new and buying used is absurdly compelling. I just saw a McIntosh MA6500 on a French site for 2700 euros, negotiable! I mean - you can't buy an amp that is one tenth that good, new, for £1800. Just saying.