want2workinuk, a US national crossing the UK border without prior entry clearance may get questioned by the immigration officer about the intents. If they have had a previous visa refusal, depending on when that refusal happened and why the visa was refused may make the IO suspecious of the person that they are simply trying to work around the process by showing up as a visitor.
For whatever reason, they assumed the OP was going to work in the UK and refused them entry. Whenever you get denied for a visa, it won't automatically cause you problems trying to enter the UK, but anyone who has been denied a visa should ask themselves honestly "are they going to think I am just a visitor if I show up at the border without prior clearance?" If you think you might get refused, because it has been a short time since your refusal and your personal circumstances are similiar to when you applied, you should consider seeking entry clearence.
Once you get refused at the UK border, it is very unlikely that you will be able to enter the UK again without prior entry clearence. As noted in this thread, the OP will likely need to over prove they are nothing but a visitor. Those that have been refused but apply for a work or family visa though, assuming everything is in order and they meet the requirements of the visa will seldom be denied the granting of that visa.
