Upvote:7
The UK operates a regime that does not recognise Italian residence visas as travel documents. Instead they go solely by what kind of passport you have. Assuming that in your case you have a Filipino passport, you would need a visa.
For the other part of your question, the rules will allow you to officiate as a pastor at a one-off event like the anniversary celebration you described (Paragraph 15, Permitted Activities, Appendix V).
You need to apply for a Standard Visitor Visa, and the Visa4UK site will help you get started.