Welcome to the Community, Manesh.

I'll be happy to share the solutions.

First, let's make sure that your multicurrency feature is turned on by following the steps below. Note: When the feature is enabled, turning it off is unavailable. 

1. Go to the Gear > Account and settings.
2. On the left panel, click Advanced.
3. In the Currency section, select your home currency from the drop-down menu.
4. Select the toggle icon to enable Multicurrency.
5. Put a check mark on the I understand I can't undo Multicurrency box.
6. Hit Save and then Done.

Here's a sample screenshot for a visual guide:

Next, create a new dummy bank account for the home currency. This is to facilitate the movement between 2 different foreign currencies.

Here's how:

1. Click the Gear > Chart of Accounts.
2. Select New.
3. From the Account Type drop-down, choose Cash and cash equivalent.
4. Set Cash on hand from the Detail Type drop-down list.
5. Enter Dummy Bank in the Name field and make sure to select the home currency (which is SGD in this case) from the Currency menu.
6. Hit Save and Close.

Once done, open the invoice and record the payment. In the Receive Payment page, select the Dummy Bank account from the Deposit To field. Before saving the transaction, take note of the home currency (SGD) amount displayed at the bottom right.

Finally, transfer the money from the Dummy Bank to the GBP bank account. You can read this article for reference: Receiving and making payment for a foreign invoice/ bill with a foreign currency.

Please let me know how things go after recording the transactions. I'll be right here to help you out some more. Take care and stay safe!