I would not let it slide. It does not sound right, under any circumstance.
Absolutely NO WAY let this slide.
Your worst nightmare is that they trash the property, pull out the sale, then through a third con person, renegotiate a cheaper sell.
Get evidence, make it legal by reporting to police and solicitors tonight.
i) Complete a 111 online police report TONIGHT - be very specific and non-dramatic. "We are due to complete on this property tomorrow, however it appears that the buyer has broken into the home and vandalised the property." You are just covering your back (for legal and insurance purposes) by stating that your permission was not sought or granted.
ii) Email your solicitor TONIGHT. He will act quickly because a criminal act is occurring. Both solicitors will also be motivated to get the completion sorted asap.
iii) Ask your soliticitor to email them first thing accusing them of tresspass and vandalism, with accompanying photos, and state if the completion does not occur by midday tomorrow, you will be holding them criminally liable.
iv) Be at the property 7am with phone (for film evidence it's them who have trespassed the property). Politely but firmly insist they (or their contractors) leave the property until the sale has been completed. You can be polite and friendly, just insist they leave until the sale is complete. Just keep repeating "Under legal advice, please leave the property now".
v) You DO NOT want them to allege that you verbally agreed to this - and that is what it "may" look like if you do nothing.
And then go get some counselling or life coach lessons. Being "nice" to people does not mean they will be "nice" to you.
They are behaving criminally and you have a solicitor, who you are paying for this house sale. If you're ever going to grow a back bone and stand up for yourself, now might be a good time to try it. You've all got too much money invested in this property.