I would just say 'I have a child with SEN and we're waiting for the proposed Statement to come through. I'd like to meet with you to see if you could meet DS's needs.'
You'll be able to tell a lot just from their initial attitude.
I visited 3 MS schools and the Special School when DD1 was being statemented.
MS 1 -kept us waiting 20 minutes in the foyer, very sniffy, waved dismissively at the SEN area, warbled on about school hall upgrades, etc. Very obviously all targeted at the MC parents who were viewing along side us. Put the onus on me to get the best statement I could 'or they wouldn't be able to provide for her.' Ignored DD1 pretty much, while shaking the hand of 'Eleeeeaannooor' and saying 'let's see if we can find a school for you.'
MS 2 - couldn't fault them in their attitude. Waiting for DD1 at the door, led the tour by her preference, focusing on her. Asked me to come and meet the SENCO so we could chat about what she needed to make transition easy, etc.
MS 3 - very experienced with SN, high number of statemented children, aggressive attitude towards pursuing support with the LA, but I felt that the HT was a bit too 'knowledgeable' and had a 'been there, done that' attitude, so I felt she would think that she knew best for DD1 and we wouldn't have much say in her education there. Rightly or wrongly...we'll never know.
Special School - wonderful HT, totally focused on DD1 and assessing even as we talked. Totally unflappable. Safeguarding DD1 even as we talked - guarding table edges distinctively as she wandered around the office, etc. Using makaton as she talked, effortlessly including DD1 while maintaining dialogue with us.
Ironically, MS 1 is the school I've moved DD2 to, despite hating it for DD1 and refusing to even put it on my preference list when she was entering Yr R (the HT has changed, so perhaps that explains the difference now).
MS 2 is the school I've removed DD2 from. The HT changed the year that DD1 was entering YR R, so I dealt with the Deputy Head and the old HT when discussing DD1.