I am an indian national, currently staying in USA. My current H1B Visa has expired and I am planning to travel back to india via London, UK.
Will I require an airside transit Visa for London?
Assuming you've been in the USA for some time on your H1B visa, then yes, you will need a visa to transit through London in your case
You’ll need a visa to pass through the UK in transit (unless you’re exempt)
Next, you want to look up the "Exemptions" section on the visa check page (it's near the bottom). From there, you'll see
You won’t need a visa if you: ....
are travelling to Australia, Canada, New Zealand or the USA and have a valid visa for that country
are travelling from Australia, Canada, New Zealand or the USA and it’s less than 6 months since you last entered that country with a valid entry visa
So, on your way to the USA, you need to have a valid entry visa for the USA. On your way back from the USA, you need to have entered the USA within the last 6 months on a visa that was valid at the time. Having entered 7 months ago but with a visa that's still valid isn't allowed, though having entered 5 months ago with one that is now expired is fine. You can find some more details, along with worked examples, in section 2.6 of this guidance document for airlines.
Since an H1B is normally valid for quite a while, I'm assuming you last entered the USA more than 6 months ago. Assuming so, since it was more than 6 months ago, you won't qualify for the 'Transit without visa' concession. Thus, you'll need to apply for a Direct Airside Transit Visa. Well, either that, or leave the US less than 6 months after your last entry!
I am facing the same dilemma right now. However I seem to have the answer for my own (Indian travelling back home through Heathrow) but with a valid US visa. Please check if you need the transit visa on this link. That link can be used for people from different countries and in different situations. Basically the answer has already been given in the link given above for "UK Border Agency Do I Need A Visa tool"