What you are planning to do is perfectly legal. There is no requirement to buy any ticket before entering the Schengen area and nothing in the official regulation that would forbid buying tickets in Amsterdam and staying in the UK in the middle of a 90-day Schengen stay. Only the days during which you are present in the Schengen area (based on entry/exit stamps) count toward the 90-day limit so as long as you are not staying longer than 90 days in any 180-day period, splitting the stay in two is not a problem.
At the same time, having all the relevant tickets when boarding the flight and crossing the border would help alleviate any concern that you are planning to break the rules. Border guards might ask about that but if you have a credible plan, it should be fine. The airline will also want to check your visa back in India, before you even reach Amsterdam, and it's likely that airline personnel (or, as you found out, travel agents) would not be familiar with the fine points of the rules. So if you can help it at all, I would recommend buying some tickets to the UK in advance to make things simpler.
Also note that the maximum stay is now ninety days, not “three months” so make sure you got the calculations right and you will be leaving the Schengen area in time. It would seem prudent to have a short buffer at the end (i.e. leave after 80-85 days) in case something goes wrong (a cancelled flight is not an excuse, you could still be fined or possibly even lose your visa if you find yourself stuck in Amsterdam on the last day of your authorized stay).