I use two Big Muffs onstage and they work great with overdrive, especially together. So i recommend an Overdrive.
Most non expensive overdrives are based on the Ibanez Tubescreamers circuit so... there is'nt a lot of difference in sounds.
You could go for any of these;
Boss SD-1
Ibanez TS7
Digitech Bad Monkey
Dod 250
And have change out of 30 quid, they are all decent as well.
If you want more gain try..
Marshall BB2
Marshall Guvnor
Jekyl & Hyde
MXR Distortion +
Boss OS-2
Overall, value for moneywise i think the Marshall BB2 is a great pedal. It has two modes; Clean Boost and Overdrive. It covers most sounds up to light distortion.
You'll get one on ebay for 20 quid.
One thing is for sure, i would forget Boss DS-1.