There is a difference.....
If you don't pay the extra fee you listings will be visible but unless they are relatively unique they will not show in a basic search (depending on the search terms of course) or will show up AFTER domestic listings in a section called "additional items from International sellers".
If you pay the extra fee your listings will show in basic search results along with uk based listings.
Is it worth it to pay the extra fee? It depends, if your buyers don't find you using search it's pointless, if buyers use search but use terms which only give a very few results it's pointless, if you are listing items which buyers will only find via search and those searches return many pages of results it MIGHT be worth the extra charge.
There can be a big difference between "it's possible for buyers to see your listing if they look hard enough" and "your listings will be as easy to find as any others".

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.