It is always good to know a little bit about who you are dealing with when either Major Gift Fundraising or indeed using some wider based appeal like an annual fund.
doogle.co.uk is a great site because it not only works by postcode but it pulls data from the government databases about household incomes, levels of deprivation etc. This link to Middle layer Super Output Area and Lower Layer Super Output Area data is another useful (and free) guide that helps you assess wealth for donor prospecting, assessing bursary application data etc.
So is this a creepy thing to look at?
I think it is good practice to try and understand your supporters, and so long as you dont make judgements soley on this data, and have some manual inputs and overrides I think it helps you target your limited time and resources better, then a case can be made for using it within GDPR.
It is important not to only focus on HNW individuals
It is always worth remembering that schools are communities where one time study mates may have very different life trajectories. That doesn’t mean they are not still friends. I would advise anyone to ensure their alumni networks are not only open to all but also have value for all, and value everyone in the networks. They will all be alumni of your institutions long after you have offended them and gone on elsewhere!