Hi, I have a 2013 Ford Focus 1.0 EcoBoost (petrol) with 90,000 miles. I replaced the wet belt a couple of months ago. Two days ago, while heading to work, the oil pressure warning light came on. I drove back home and parked it. The next morning, I tried to take it to the garage, but both the oil pressure warning light and the engine management light (EML) appeared.

When I opened the hood, the engine sounded normal with the oil cap closed, but when I opened the oil cap, I could hear a loud hammering sound. When the cap is closed, the sound goes away.

What could be the possible issue, and is it advisable to drive the car to the garage, which is about 30 miles from my location?

If the oil pressure warning light is on then personally, I wouldn't drive it anywhere until the oil pressure had been checked with a gauge and verified to be high enough. Plus any existing codes read based on the EML.

Beyond that, the mechanical noise may need further investigation and my concern would be that this may be very coincident with the belt change but of course, the "loud hammering sound" may just be normal so perhaps another owner can check that for you and come back on hear (pun!)?

The loud hammering with cap off is normal.  I think it's the high pressure fuel pump tappet.  It can be heard on many of them on YouTube.

I would not recommend driving the car 3 miles with an oil pressure light on, let alone 30 miles.  It could just be a loose electrical plug, but it's not worth the risk unless you can confirm that there is enough pressure.