Firstly, she probably was offering a HE for extra all along (and surely provided it for others) but you'd never asked. Depending on the girl and the establishment, HEs may not always be offered, the girl will wait to be asked. This way they keep their legit customers, but also keep themselves open to earning extra money should the customer ask.
She obviously saw you as a 'safe' regular and when talk of your birthday came up, she decided to raise the issue now that she had an excuse, so to speak. You took her up on it.
Your next booking wasn't a birthday so she went back to 'let the customer ask' mode. You didn't ask

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A bit further down the line, she got fed up waiting for you to ask, so she just did it anyway. And lying there with a boner, you're not gonna say no, are you? Presumably you paid the extra on that occasion. If you hadn't, it'd be back to just doing a normal massage for you. And so it goes, round and round in circles

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I think basically, you just need to decide what you want, and ask her for it. If you continue not asking, she'll probably wank you off every so often anyway, and should you choose not to pay her the extra £20 she'll either never offer it to you again or she'll refuse to see you.