In a word, no. A 'mortgage-in-principle' – or an 'agreement-in-principle' –  is a conditional offer from a lender saying you may be accepted,
based on a quick check of your income and, possibly, your credit file.
In a heated property
market, you are likely to be asked for one by a seller before they will
accept an offer. In addition, for first-time buyers, it boosts confidence they'll be accepted.
But it offers no guarantees.
Do be mindful that too many of these checks in a short space of time could harm your credit rating.