You should be able to request this at the check in desk or an Easyjet service desk if you check in online, it is operated by the airport but arrangements should be made through your airline and they will organise it. It's free and so long as there aren't two passengers in need of the buggy your partner will be allowed to sit with you.
On a side note, thankfully the need, if there ever was one, for disability registers were eliminated with the 1995 Disability Discrimination Act, the term 'registered disabled' is something that makes my skin crawl and should be well and truly consigned to the past.