Enter the Schengen Space if I hold an onward ticket

If you satisfy all the conditions to enter the Schengen area (visa or visa exemption, finances, insurance, etc. and, for the time being, Covid-related requirements) then it is perfectly fine to go for a short visit from the UK or on the way there. Having the means to leave the Schengen area is one of the requirements and holding an onward ticket, while not strictly mandatory, is a good way to satisfy this requirement.

Note that what Schengen border guards care about is your leaving the area in time. It's perfectly fine if you have a ticket to any other country you have the right to enter. You don't need to have a ticket to your country of citizenship or ordinary residence.

Conversely, if you do not satisfy one of the requirements, having an onward ticket won't be enough to secure entry. For example, if you would otherwise need a visa, you cannot hope to get one in Madrid while in transit (and, in fact, the airlines wouldn't let you board if your planned transit is too long). Being based in the UK and flying from there also won't exempt you from any visa requirement.

Borders guards are also tasked with evaluating the purpose of your trip. If there are signs that you do not really intend to return to the UK or might actually be trying to immigrate in Spain illegally, you could conceivably be denied entry or detained until your flight back to the UK. I have no reason to think that it is likely but it's part of what the entry check entails.

Either way, the onward ticket is not the only or main requirement and will not change much to the decision.