Sorry to hear of this.
As they are regulated by the Financial Conduct Authority, they have to comply with money laundering rules and to ensure they have anti fraud measures in place.
They clearly suspect something and this is the result - there is no quick fix unfortunately and you only have one option in my view.
Given the dispute with the bank, you can avail of the Financial Ombudsman Service dispute resolution scheme - which is much quicker than court action and free to use. You have to go through a complaints procedure first.
You have protection under the Consumer Rights Act 2015 here which means you have a right to expect a service (from the bank) carried out with reasonable care and skill. Furthermore, the account service should be “fit for purpose” and “as described”. If it isn’t then you can allege breach of your consumer rights and breach of contract.
You should now make a formal complaint and if that does not resolve the matter ask them for a "deadlock letter", which is a letter giving their final response.
You can make the complaint on the app or on their website so please do this without delay unless you have already made a formal complaint to them. If you have and 8 weeks have passed, you can contact the Ombudsman.
When you do make the complaint, tell the bank that you plan to report them to the Financial Conduct Authority (FCA) who regulates them. You can reach the FCA to report them on 0800(###) ###-####if you wish. It may speed matters up.
Once you have the deadlock letter the next step would be to escalate this to the Ombudsman - they will investigate and liaise with the bank which would hopefully result in a reinstatement of the account and full access to the funds. They can order the bank to make a financial award for inconvenience if they have acted poorly.
Once you have their final response, you can make the complaint to the Ombudsman here :
Or by email:***@******.***
The Ombudsman will look at this case independently and will make a decision based on what happened. Their decision is binding on the bank.
The above is the only way forwards in my view, short of court action. A lawyer is not required - the procedure is set up for consumers and small/medium sized businesses as a means of alternative dispute resolution.