Can you travel by tube from London Waterloo to St Pancras international station?

No, there is no direct tube line from London Waterloo to St Pancras International. However, you can take the Waterloo and City line from London Waterloo to Bank, and then change to the Northern line to St Pancras International. The journey takes approximately 25 minutes. Alternatively, you can take a taxi or an Uber, which will take around 15-20 minutes depending on traffic conditions.