The law does not differentiate between an estimate or quotation, as it is the wording that dictates the terms.
For example: "Price to remove bath, plumbing work, and replace with new bath (supplied by customer) £180+vat" will be construed as an offer irrespective of whether it was titled as an estimate or quotation, and if accepted by the customer, a binding contract is entered into and the general rule is that providing the bath is fitted to a reasonable standard, then the offeror is entitled to the agreed price; no more and no less.
But, consider this: "Price to remove bath, plumbing work, and replace with new bath (supplied by customer) £180+vat, subject to full survey of premises and type of bath supplied by customer". Although this offer is still capable of being accepted, there is a chance that the price can fluctuate at the point of billing for the completed work if (lets say), the bath was larger than anticipated.
Therefore, a builder cannot state "Estimate to build the house as per plans and specification £200,000", then come along and charge £300,000 (notwithstanding variations).