No, you can't as mediation is voluntary and requires the other party (ies) to agree to mediate. But do be aware that the court will ask why mediation was not tried and may penalise someone for not agreeing to mediate.

No. It is never too late to mediate. The court will recommend it anyway, and for claims below £10,000 you may access the free court mediation service. But this is limited to one hour telephone mediations..

Unlike the other methods of resolving a dispute (such as court or arbitration), no decision is imposed upon the parties. It is a voluntary process by which an independent mediator facilitates the parties in reaching an agreement themselves. Learn more

Do it as early as possible before legal costs and expenses are incurred. That can be after you know you have a disagreement and had an opportunity to exchange views informally.