In the last decade, the Transmission System Operator (TSO) in Great Britain has seen an acute declining trend of reactive power demand during periods of minimum load. Reactive power traditionally being absorbed by distribution networks is now in many cases injected to transmission. This not only results in significant voltage regulation challenges to the TSO but may also trigger future requirements for Distribution Network Operators (DNOs) to manage these reactive power exchanges. Nonetheless, for the TSO and DNOs to adopt suitable investment strategies, it is crucial to first quantify the extent of this decline in the near future. This work proposes a methodology to identify trends of reactive power demand using historical DNO monitoring and network data. Multiple scenarios are also considered to cater for potential changes in demand, generation and networks. The methodology is demonstrated on real British distribution networks, from the transmission-distribution interface to primary substations. The assessment reveals the significant effect of demand trends in primary substations on the overall transmission-distribution exchanges. It was also found that the potential use of shunt reactors in distribution networks would require significant investment, highlighting the importance of understanding reactive demand trends in planning cost-effective solutions.