Metal flask while travelling from Heathrow Terminal 3 to Asia

The flask is fine, the potential problem is the water in it. Liquids over 100mL are generally not allowed, but exceptions can be made for baby food.

The easiest solution would likely be to ask cabin crew for hot water, they're usually happy to oblige. They can also heat bottles and food for you.

Last but not least, and straying into Parenting.SE land, there's really no reason to insist on boiled water, bottled or tap water will do just fine. If your baby is 10 months old, they're already getting way more bacteria from crawling around and inserting random things into their mouths than they'll ever get from clean tap water.

So, in general, liquids must be carried in containers of less than 100ml. However, there is an exception for baby food etc on the Heathrow page you link:

Can I carry liquid baby food, prepared milk, soya milk, rice milk or sterilised water through security?

Yes, if you are travelling with a baby or infant you can carry a reasonable amount of liquid and liquidised food for the baby or infant infant for your journey. To assist security screening, please do not carry these items in a flask. Spare supplies must be packed in your hold baggage.

You can either carry the liquids as part of your liquid bag allowance in which case the standard liquid rules apply or alternatively separately in containers over 100ml. Following x-ray screening, security officers may also need to carry out additional checks on these items.

It doesn't specifically say that you cannot use a flask, it instead requests that you don't. It's possible that a flask will be taken off you, or possible just that it causes additional delays/hastle with security screening. You should be prepared that security may wish to take a sample of the contained liquid for analysis.

However, if the only concern is the provision of boiling water, than the cabin crew of the aircraft should be happy to provide that for you. While water boils at a lower temperature at altitude, at typical cabin pressures the water should still be at 91-93°C, more than sufficient to kill any microbes.

As jpatokal said, the metal flask will be fine, but the water inside probably not. You should take the flask out of the bag, otherwise it will look suspicious in the X-ray. I have been allowed to keep the hot water in the flask when I forgot to empty it before the security check (babies...), but I wouldn't rely on it.

You can probably find hot water from a water cooler inside the terminal. They have gotten more common as Chinese travellers prefer hot water. It may not be boiling hot. Otherwise try asking a cafe in the terminal. They have boiling water for tea, and if you say it's for baby food they will probably fill your flask for free.

Then, as others said, the cabin crew on board will have boiling hot water for tea.