I do not have access to look at ukpn network maps. But I would have thought there should be no issue in you asking UKPN for either a quote for a 3 phase supply or most utilities will give out free maps for digging purposes anyway.
I would ask for a quote as there may be 3phase available. But the transformer may be at its limit and need upgrading for what you require or it may not??
If there is overhead in the feed to your under ground cable it may be 3 phase or split phase or just single phase. Split phase is easier to convert to 3 phase than single.
You don't have to use a DNO to do the digging or even the connection. You could do the digging on your land and I believe you can get the connection done by an independent network operator. As your in the ukpn area there may be independents that cover your area. Something you could look into?? But ask UKPN.
Ok but if you speak to the local office that covers the area and explain that you are buying the property but don't own it. Then they might still have a quick look at the maps. If all you require is a 3 phase 100A cut out then it will be fairly easy for them to say yes or no to the cable being 3 phase in the road or if the transformer is at its limit. If it's not even 3 phase to start with then you will know the work is going to be more involved.
I can't imagine they will charge you to look this up. You need to speak to one of there planning staff who plans service alterations which is what you are after.
Oh something I forgot to mention. Have you checked that it is ukpn network ?? Look on there website, they should have a post code or address checker. What is becoming more and more common is private networks. As I said you don't have to have the DNO do the connection. And more housing estates are being done by private contractors. These are then private networks within the DNO network and may or may not have been adopted by the DNO. If not it may not be serviced or maintained by them even though it's in there geographical area.