Sorry to hear of the issue.
If you withhold service charges there is a risk of forfeiture of your lease (your lease is a contract and it will say you are liable to pay service charges).
You can however ask your local First Tier Property Tribunal to make a determination as to the reasonableness of your service charges. This is free to do and only takes a few weeks.
It is one form which you will need to complete and send by post or email to your regional tribunal.
The tribunal will then make a decision and indeed it has the power to appoint or remove a management company for another one to ensure good management of the property.
The Tribunal is made up of 3 people: A lawyer (Chairman), a surveyor/valuer (who could also be Chairman), and a lay person (member of the public).
You can present the dispute to them in person or use a law firm (such as either of these law firms : https://www.bishopslaw.co.uk/first-tier-tribunal-property-chamber-advice/ or https://www.bradysolicitors.com which has specialist department for these Tribunal claims).
The latter firm also offers a free guide on how to change managing agents : Click Here
The hearing itself is quite informal.
Once made, the order is then binding - if the order is ignored then the county court will recognise it and you can apply for an injunction to force them to comply.
You will need to fill out the relevant form and then email it to your regional Tribunal. The Tribunal should then make a decision within 6-8 weeks.
The form to use is this one :
Your regional tribunal would be listed on this site : Click Here & Scroll Down