Did you go to musicroom.com for it? I am from the UK too and ordered from there right after I saw it on sheetmusicplus. I initially thought I would cancel/return the sheetmusicplus one but the UK one didn't come for ages so I cancelled that order. Received the US one in a few days though. Maybe it was just an oversight for me. I would go for the UK if possible.
Given what you play you shouldn't find it too hard. It is one of the hardest pieces I have learnt so far (the other being a Chopin Waltz) but I am maybe just slightly below you in standard (doing ABSRSM Grade 8, but hoping to do the Lark for a diploma as an own choice piece). It is probably DipABRSM standard, maybe higher if you are comparing it with the LCM, TCL syllabuses.
The hardest bit notes-wise in the Lark is for me anyway, the middle section with all the scales elaboration of the main theme. To match the hands and have the right hand light as an accompainment when the melody is in the left hand. The first 2 1/2 pages are pretty easy note-wise. I plan to start on the last two pages during the next couple of weeks. I sight read them and it shouldn't be to hard to get the initial hang of.
I think what is hard about this is actually getting it polished musically once you get the notes (I haven't yet) and to get the "ad libitum/free time" stuff really fast and smooth. I don't know what you call them, those smaller sized notes without bar lines.
Another thing is I don't do piano as a career/degree, so what I may find difficult might be easy for some, certainly it would be easy for say Eddie.
Good luck and enjoy it. It is a lot of fun to play and out of the 7 piecews I have on at the moment, its my favourite (though I love all 7).
BTW, since you are from the UK, you probably heard Benjamin Grosvenor play it? It was him who got me wanting to play it actually. I have Kissin's rendition of it, which is beautiful. It's an mp3 if you would like me to send it to you? PM me if you do anyway.
Topic: Glinka-Balakirev, "The Lark" sheets