The "Nuclear Triad" is more of a historic artifact; the result of several years of organizational rivalries that eventually ossified in this form, rather than something which was deliberately engineered to be this way from the start. The necessity of strategic nuclear bombers is particularly suspect, but for a time they were the only option to get the job done and for that reason SAC was very politically powerful. Furthermore, the deterence factor from land-based ICBMs is relatively weak (although this can be improved by making the missiles mobile, overbuilding silos, then playing a shell game with them.)
If what you need is a credible "we'll ruin you from beyond the grave" threat, then the UK's four SSBNs get that job done well enough (presuming the chance of the missiles working is high enough to not take the risk.)
