It might be easier to establish the historical budget figure.
A written question by Lord Burnett asked Her Majesty's Government some budget questions in April 2017.
The answer by Lord Howe gives some insight into obscure Defence budget protocols.
I'm advised the projected Navy budget covers ten years, hence the reason you'll only see retrospective accounts as past spending and saving will influence the amount available in future. For this reason, the specific figure for say 2018/19 cannot realistically be asked at AIB unless the person asking is clairvoyant.