You are a British national and your wife-to-be is Uruguayan, a non-visa national. You want to travel to the UK with her for a visit.
A spouse visa is not required if they do not plan on settling in the UK; it's the wrong visa for visiting. A non-visa national who intends to maintain family relationships in the UK would apply for leave to enter as either a FAMILY VISITOR or GENERAL VISITOR. People do this all the time.
Having said that, if your wife does not have a credible peripatetic history, or it is her first visit to the UK, the Immigration Officer may (or may not) conclude that she is attempting to evade controls and remain in the UK. If that happens, she will either get a temporary admission or she will be served with a removal notice at the airport.
If you are not certain of the outcome and wish to avoid the stress of removal at port, she can apply for entry clearance as a FAMILY VISITOR in either Uruguay or Chile. If it is granted, there will be no problems landing and entering the UK. Note that there is no requirement for Uruguayans to obtain an entry clearance, and this step is undertaken for expedience and comfort only.
"...The rate of refusal of leave to enter at the port to visitors from
some non-visa national countries ... is higher than for visitors from
other countries. It can therefore be advisable for nationals of those
countries to obtain entry clearance, and the rules permit though they
do not strictly require it (para 23A). Even before the Immigration
Order 2000 SI 2000/1161, entry clearance would ease the traveler's
passage through immigration control and help to avoid wasted airfares
and the distress of refusal at the port..."
(source: "Immigration and Asylum Law", Gina Clayton, LLM, Oxford
University Press, 2004).
To summarize, your wife has two options...
Apply for leave to enter upon arrival using her Uruguayan passport; or
Apply for entry clearance as a FAMILY VISITOR in Uruguay or Chile.
The choice is hers and should be the one she feels most comfortable with. And a spouse visa should only be applied for if she intends to enter and settle in the UK. It is not needed for visits.