Have you asked migrationsverket in Sweden. They will be able to confirm whether any additional documents are required to travel within Europe.
UK is not part of Schengen.
If your wife travels on her Thai passport she may need additional pappers as UK may not accept the Swedish ID card as the country is outside Schengen.
Best to check with migrationsverket / Swedish embassy if she will need extra papers to travel to uk as it outside Schengen.
Did you check with the Swedish embassy / migrationsverket?