Hello Imsosorryalan
Was in much the same position as you for DD1 (grammars or less than stallar comps - now both in special measures).
Our solution was up the work at home for DD1 (who finished Y2 on NCL1 on KS1 SATs) and move to avoid 'sink' schools.
If you can't move (for financial/ personal reasons) then really put in that time at home.
Several things can make a huge difference at ages 7 - 9:
This is the time to ensure that reading skills are high and that vocabulary is developing. Really encourage nightly reading - it makes a huge difference. Read to your child - don't drop this - read books that they are interested in but can't access yet or read to them at weekends from books you both enjoy (your childhood favourites, series - Lemony Snicket is great fun is you like dark humour, etc...).
Work on those calculation skills: addition/ subtraction/ multiplication - you really want them capable of handling all of this to 3 digits & really 2/3 decimal places. With division - at this age - really aim for by 9 able to handle inverse multiplication facts - so 36 divided by 9 is ? And instantly knowing it's 4 because of the 9s times table facts.
If you work on that for the next 2 years - by Year 5 (the year you seriously have to prepare for the 11+) you should be in a good place. We didn't start seriously working on preparing for 11+ until Sept of Year 5 - starting with things like bond books/ free downloaded materials off 11+ websites. DD1 actually did pass the Birmingham grammar test to get into one but it was quite far away (1 hour commute) and our local option (having moved) was good; so we agreed with her decision (she didn't want to do buses on her own) to go to the local comprehensive.
Finally I'd advise following discussions for your region on the 11+ forum - usually at the top of the regional page there will be discussions on best resources, typical content of past tests, etc... which can help you guide preparations in Year 5 (ages 9 - 10) toward success on the test. Link here: www.elevenplusexams.co.uk/forum/11plus/index.php - just scroll down to regions and find yours.
HTH