I'm hoping this is a useful guide for anyone new here and is looking for a cheaper starter pen. All the pens I have so far, I got for < £30 (might break this in the future for a TWSBI Eco/TWSBI Diamond 580 but that's another tangent). Putting my honest thoughts down. Please feel free to correct any info I got wrong here, add more info, or if you have a differing opinion on any of these pens.
Kaweco Perkeo - Price in the £10s. Good full size starter pen. Converter friendly. Comes with a fine or medium nib. I love these pens but sometimes the nib isn't great out of the "box". I find the medium nibs a safer option in terms of smoothness. Nib is removable for cleaning so switching inks is super easy (I love disassembling and reassembling my pens).
Kaweco Sport - Price in the £20s. Pocket pen. Lid screws on. Nice length when posted. Super lightweight. So many nib options. If you like weighty pens, you probably won't like the plastic version. I have small hands so I love these. Again nibs are a hit or miss so something to be aware of.
Parker Frontier (my first fountain pen) - Price in the £20s. Solid pen. Nib is quite broad for a medium. I mistreated mine so much because I had this before I knew how to care for fountain pens (pretty sure I cleaned it with nail varnish remover at least once) and it's still working superb.
Lamy Safari - Price in the £20s. Plenty of colours. Plenty of nib options. Good value for a fountain pen. Writes a little dry. Medium nib is a bit broad. Again had this before I knew fountain pen care. Probably also cleaned it with nail varnish remover at least once. Working fine still.
Søstrene Grene Fountain Pen - Price < £10. not sure if it's still stocked. Very cheap. Metal body. Don't know what converters fit so have been refilling the cartridge it came with. Don't know nib size. Skips a little. Varies heavily based on ink.
Platinum Preppy - Price < £10. Personally not the biggest fan. It's a good starter pen especially for the price (it's the cheapest here) but feels a little flimsy compared to the rest.
Faber Castell School+ - Price < £10. Cheaper pen but gets the job done with occasional skipping. Nib is quite broad for a medium but gets the job done.
Pilot Prera - Price in the high £20s +. This might not be within £30 normally. Pocket pen. Slightly bigger than the Kaweco Sport and comes with a full size converter so holds more ink than the Kaweco Sport. Very smooth writer even with the fine nib and drier inks like Pelikan 4001 black. I found mine smoother than my Kaweco fines.