This is very much an “it depends”...the Ops folk will have an ideal plan for which aircraft on which sectors several days out but if the plan for Saturday involved an aircraft that has suddenly gone sick there might be some rejigging going on and the company might decide to declare delay unknown or unquantifiable until they have a fix...so it is possible you may not have a delay after all.
It’s also worth worth considering that:
1. There generally aren’t many if any spare airframes and crews.
2. Longhaul aircraft leaving LHR may not get back to base for quite some time (12 to 48 hours plus) and so “reactionary or “ knock on delays” can be known 24 hours or more in advance.
Hopefully this far out I’d suggest there’s a chance of a fix and your flight will “come good” and there will be no delay..Good Luck.