So it's almost the end of year 11, and my English skills are not up to standard... I have no trouble coming up with ideas for essays, I just can't put my thoughts onto paper! I'm just generally a slow person so when it comes to writing the essay I get so muddled up to the point where my writing isn't clear and I go off topic. Has anyone else experienced this?
Help!!
Yep, it's pretty tough issue

Be glad though, you honestly do have time, if you're willing to attack the issue head-on

Maybe try this! Make a dot-point plan like normal; you say you have ideas, so you can put them down in any words even if it sounds stupid. Then order it, and then make it more and more detailed, by adding in quotes, ideas, etc., until you have
one brief dot point for each sentence that you'd write in a final essay. Because it's a 'dot-point plan', you don't have to stress at ALL about how it's written, if it's written in a grammatical full sentence, or anything.
Then, turn each dot-point into a full, proper, flowing, sophisticated sentence, one at a time. Don't expect perfection, either.
It's very time-consuming, but after a couple you'll be able to get from a slightly shorter plan (e.g. one dot point per 2 sentences) into a full essay, and so on, until with practice, you'll hopefully be able to write off a normal brief para-by-para plan!
Just doing this will really help you get used to expressing your thoughts without you getting lost, and you can gradually build up confidence.

(NB Since you can't do this in exam conditions, it won't help you with y11 exams or SACs, but in between, e.g. over the huuugggeee holidays, you really will have time for an occasional bit of practice so you're more prepared come year 12 SACs).