Viva Street ads are no guarantee of anything at all. Even your link does not get you back to the ad you are trying to talk about. A screenshot (with number displayed) may have helped. The ads can list extras and fees and thereby a guide to whether extras are on offer. Alternatively some SPs don't offer extras at that stage, or even on 1st WhatsApp message but may offer in the room....such is "will she won't she".

The link works for me.
I realise there's no guarantee of anything in life but there are certain signs that look suspicious or 'less legit' to me.
For example;
The decision to advertise in Vivastreet (under Slough Escorts & Massages) when there are plenty of legit massages directories out there.
The fact that they specify the nationality of the masseuse (Indian) - if it was a legit massage you wouldn't really care about the nationality or the appearance unless you were specifically looking for a Thai massage.
The age of the masseuse (in this case 19) - for the same reasons above.
The fact that they're open from 10am to 9pm - unusual for legit places.
It wouldn't surprise me at all if extras were available.