Sir, I lodged my file on 21st May only, actually I forgot to mention my previous visa refusal, once I realized I forgot about the past refusal, I immediately raised a web form to inform them, to add past refusal in the visa application, and later on I raised the request for the withdraw the application.
Kindly suggest what would be a good approach. I feel that if I go for the biometrics, the application will be moved forward there would be no way to step back and get it canceled or withdrawn easily.
As mentioned earlier, if you don't complete the biometrics request within the provided timeline, your application will be refused. This refusal should be related to biometrics(1) and not misrepresentation(2) unless they already evaluated that too.
To process the withdrawal request they probably need the same amount of time or less. If the withdrawal is not processed before the actual application and it gets refused for (1) or both (1) and (2), you will have to mention these refusals in future applications.
Since it seems like you will have a refusal to report anyways, why not go ahead and give biometrics? Wait till the very end of the deadline and if your withdrawal hasn't been processed, submit your biometrics.
Another alternative would be to submit a new application. Include a clear letter stating you have submitted a withdrawal request for the 1st application and why you wanted it withdrawn in the 2nd application. Submit a webform for the 1st application informing them of this change.
That's all I can think of. All the best.