Nikki decided to learn singing on a lark.

Is the idiom "on a lark" used in British English? Have I used it correctly in these sentences? Are they okay?

1. Nikki decided to learn singing on a lark.
2. Gareth started the website on lark but it developed to be a very successful business.
3. The billionaire bought the NBA team on a lark and sunk two billion dollars into it.
4. Jane bought a 75-inch TV on a lark but experienced buyer's remorse when it was delivered.
5. I asked her out on a lark and wanted to just fool around with her, but things got serious and now I'm thinking about proposing.
6. I enrolled in an acting class on a lark. Little did I know I'd end up performing in the same movie as Susan Sarandon.

I've never heard it. All of your sentences sound to me as if you've mixed up "as a lark" and "on a whim".

In American English it's either "on a lark" or "as a lark". (Merriam-Webster.)

No. 2 should be "developed into a successful business".

No. 3 doesn't work for obvious reasons

I’m more used to hearing ’for a lark’.

Tarheel said:

No. 3 doesn't work for obvious reasons

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They are not obvious to me, apart from 'sunk' for 'sank'. Could you explain, please.

5jj said:

They are not obvious to me, apart from 'sunk' for 'sank'. Could you explain, please.

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You don't spend a billion dollars on a lark. That's serious money. Also, being a billionaire means your net worth is a billion dollars or more. It doesn't mean you have a billion to spend. It might be tied up in investments. Also, if I'm a billionaire that means I'm worth at least a billion dollars -- not two.

Those objections to #3 are subjective opinions @Tarheel. From a language point of view there are only two problems, 'sunk' and (for speakers of BrE) 'on a lark'.

OK

I've heard "for a lark" and "as a lark". I'm not sure I've heard "on a lark".

alpacinou said:

Gareth started the website on lark

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You need "a" before "lark". And I'd use "for a lark" here.

Barque said:

I've heard "for a lark" and "as a lark". I'm not sure I've heard "on a lark

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The compilers of Merriam-Webster have, as Tarheel noted in post '3.

I don't think (as an AmE speaker) I've even ever heard the versions with 'for' or 'as'. I've only heard and used 'on', although I see Webster does list 'as' as an alternate form of 'on'.