A few years ago this would have been quite a common practice, but not anymore. It is important for you that the correct payment is shown on in the records, three main reasons
1. you pay ITP (purchase tax) on the price paid, but the tax is actually due on the value of the property, so you will be paying too little tax. When the sale price flags up at the land registry as well below expected market value the tax people will want the extra tax, so you will have to pay complimentary tax. The whole point of this complimentary tax was to stop this type of practice.
2. when you come to sell the property, if not your main residence, you will pay capital gains tax on the 'profit' you have made. The figure used for the purchase price will be the one on the records, so you could be appearing to have made a lot more profit than you actually have, therefore will pay more CGT.
3. It's dodgy practice! You are signing an official document knowing it is inaccurate.
So tell your solicitor you are not agreeing to this, and put it in writing. This isn't a matter for the estate agent to railroad but for the legal people to get correct. If you are present for the completion at the Notary you can personally check the documentation is correct.