You might have opened non-base currency balances which limit the amount you can withdraw in a given currency. You may refer to the following link for instructions on how to convert these balances back to your base or other currencies: How can I convert currency at MEXEM? If your account is a cash account and you recently executed a closing trade or currency conversion, you are required to wait for settlement before you can withdraw funds.
This takes generally two (2) business days for stocks and for currencies (T+2). If you recently deposited funds to your account, please be aware of withdrawal restrictions that may apply to your recent deposit. If you have open the -F account UK Regulated used to trade non-US index options and spot metals, you may have cash balances in such account that have to be transferred internally to the main account first.
You can see cash balances in the -F account in TWS Account Window, section Market Value Real FX Balances, column "FX Cash"; you can also see these cash balances in your activity statements under the Cash Report section in the "UK Regulated" column.
You can launch an internal transfer by logging in to Client Portal and selecting the menu Transfer & Pay followed by Transfer Funds. Click on the account number in the upper part of the page and choose the account ending in F. Then pick Internal Transfer and choose Full Transfer in the Currency drop down.
Click Continue, confirm by entering username, password and the code sent to your email and click Continue. The transfer will be reflected on the account in an instant.
See also: How can I check my cash balances in the Activity Statement? and Are there any limits to the amount I can withdraw?