A Lasting Power of Attorney (LPA) can have immediate effect if you specifically want this to be the case but otherwise, no - making an LPA will not mean you lose control over your affairs straight away.
LPAs allow you to choose and appoint one or more people to make decisions and act on your behalf concerning your finances, property and personal welfare (such as health care) if you ever became to ill or lost the mental capacity to make such decisions yourself. They are extremely important documents because medical, care and financial professionals are increasingly asking for valid LPAs before they will discuss an individual's affairs with anyone else.
If you have ever become ill or had a serious accident and hadn't already prepared LPAs, your family or solicitor would have to apply to the Court of Protection to be appointed as your deputy. Not only is this sometimes a lengthy and complex procedure, the Court would need full disclosure of your finances and would also charge annual fees for supervising your affairs.
So be preparing LPAs while you are in good health can be extremely helpful to your family should the circumstances ever arise that they need to look after you.