In broad brush terms you can
try swapping any trip with anyone on your status who is agreeable, regardless of days and timings, but both swaps have to still be a legal fit on the receiving lines and that has become more difficult under EASA...You will generally only find out if a swap is permissible when you try to action it though the rostering system (eMaestro). That's when you find out if the computer says "no".... or not.
Especially if it's a swap of flights operating on the same day. Say one flight with a duty start at 9am, swapped with another at 9pm.
More info required - the outbound sectors depart LHR on the same day, but may well return into LHR on different days, even if they are trips of exactly the same length (i.e. time away from base). On a tight roster if moving the return into LHR back by 12 hours puts it into another local day there might be a conflict with EASA Minimum time off at base prior to the next trip...
In short every swap is different, you have to run your plan through eMaestro every time to see if what you're planning is allowable.