Complex sentence: One key part of the agreement was that the UK would develop their own warhead production capability for the missiles, as the UK military and political establishments were rather worried about losing their own nuclear production and design capability.
Split into simpler sentences: One key part of the agreement was that the UK would develop their own warhead production capability for the missiles. This was considered to be a key element of the agreement, as the UK military and political establishments were rather worried about losing their own nuclear production and design capability.