In middle grades, there are a multitude of web sites on the Internet. You can often find something for free that can be adapted better than paying for a book. This site, enotes, has document exchange with great lesson plans and worksheets. You can also go to a search engine and type in the specific topic such as "finding dependent clauses" or "solving one-variable equations" and you'll find plenty of resources, often free. I also sometimes limit the search by adding PDF, because those are more likely to be classroom-ready worksheets.
The grammar resource that I use in my classroom is called Daily Grammar Practice. This is a great way to use about 10 minutes at the beginning of class for total grammar immersion. Rather than focus on meaningless sentences out of some enormous grammar book this focuses on one sentence throughout the week, and identifies and teachers a different grammar component each day of the week using the same sentence all week. This scaffolding helps students make more sense of something difficult for them.
There are many on-line sites that you could use for supplemental math and english work. As someone else mentioned your local book store will also have workbooks that are good to use.
You might also check in your local bookstore in the homeschool section. If you are looking to supplement your child's skills and provide practice for him/her, this would be a great place to start for workbooks and practice exercises.
Do you have a homeschool group in your area? You might give them a call and ask some questions. They may even have low-cost and free suggestions for you in your area.
I use teachit.co.uk as well and can second kiwi's suggestion. They have a wide range of resources that you can access. Another British website that you might find useful is:
They have a massive number of different resources both for literacy and numeracy, so well worth checking out!
For English resources I would recommend teachit.co.uk. It is possible to subscribe and buy their Word and Smartboard resources, but their pdf files are free. I find the site invaluable as a springboard for ideas as well as resources.
I think that you might need to specify more of what it is needed in both disciplines. There are so many concepts for middle school children in Math and English teaching that more specific contexts would provide better and more directed answers. Having said that, I would actually examine the websites offered by the textbook manufacturers. If you are at a school that is using textbook instruction in Math and English, I would examine the textbook manufacturer's websites because much of what is in their text is mirrored with the online resources. It can help provide a seamless instruction model. Outside of that, the world wide web offers so much in terms remediation that I think you might have your pick of what is out there. I think that focusing on what you need and what your students need will be of vital importance in making a decision.