FOLLY: Don't do a buffet for your wedding, please.
Crust & Cream is a popular wedding venue for small weddings. By popular, I know of two couples that have gotten married there. One of them personally told me it was inexpensive so you should definitely look into it. My second suggestion is Arabesque, as the grounds will be the perfect backdrop for your wedding pictures. Congratulations!
NOSA: There's Monty's Buffet on Karimu Kotun, but I've never been so I can't tell you how good it is. Buffets are mainly a hotel thing in Lagos, to be fair.
If you really want save on catering/venue, check out Social Place. The owner can sort you out on food and getting married on the lawn could be cute if you work it right.
FOLLY: Check out Sao Cafe in Lekki, the outdoor space will work for you. They don't have platter but they have a few appetizers e.g. gizzard, snail, chicken wings and yam chips that you can combine together to create your very own platter.
NOSA: Or Liquid Hub in Ikoyi