Can aircrew order me not to stand in the aisle?

Yes, if aircraft safety or your personal safety requires it, you will be asked to sit down and close your safety belt. That is true for all aircraft and any time of the year.

Maybe they will allow you a bit more leeway because they understand that prayer is important for you, but if you do not belt in when it is dangerous, they endanger all passengers, that goes above your personal freedoms to pray.

Talk with the person giving you religious advice on how to adjust or handle praying when not allowed to stand, as it might be needed.
There are standard instructions for muslims who for several reasons can not stand to pray, you will likely even find those on Internet these days.

Yes. If you stand up while the seat belt sign is on you will receive a sharp rebuke! If you remain in the aisle in the way of people moving about, you may also be asked to move out of the way or to sit down.

However, many people do stand up during the flight and as long as you don't get in the way, it should not be a problem. You may be able to find a quiet area near the rear exit doors, but that's also where the toilet queues are.

Unlike American carriers it is fine to go and congregate in the galley for example.

If you are concerned it might be wise to have a quiet word with one of the cabin crew (once they have finished their meal and drink serving duties) to ask if there is a quiet place to stand, just so that the cabin crew understand what you are doing.

You wonder if you can use the aisle for praying in a British plane, just because you assume it's unlikely there will be many muslims on the plane. However it's reasonable to expect that somewhere there are some flights where the greater part of the people will be muslims. In that case, there just isn't room for everyone to do their prayers.

You might ask to any muslim flight company what happens in that case, and I'm pretty sure the answer will be either:

• everyone just prays where they are seated, or
• everyone delays their prayers

I very strongly doubt they have planes with huge aisles just to allow everyone to do their prayers, and I don't think (though I can't completely rule out) they just do not schedule any flight during those occurrences.

Sorry to be a bit harsh, but I'm afraid the other answers are too condescending, and I sense your question is very confrontational, as in "see if they dare to prevent me from doing my prayers!!!11!". If I misread your intention I apologize.

If doing those prayers properly is too important for you, and if you are sure that your religion doesn't consider this situation (I strongly doubt that), then my suggestion is to reschedule your flight. You shouldn't want to be the trouble-passenger who prevents everyone from walking towards the bathroom until he has finished praying…