Phoebe did not attend Ross's wedding to Emily in London because she was in her third trimester of her pregnancy. She was not allowed to fly in a plane due to being at the end stage of her pregnancy. She was pregnant with her half brothers triplets as he and his wife were unable to conceive so she was their surrogate.
Phoebe did not go to London for Ross' wedding in the show Friends because she had a prior commitment to attend a singing competition in New York. She also felt that she should not be a part of the wedding due to the fact that she had previously been in a romantic relationship with Ross.
Phoebe didn't go to London for Ross' wedding because she was pregnant and could not travel due to her condition. Also, she didn't want to be away from her husband, Mike and miss out on important moments of her pregnancy.
Phoebe didn't go to London for Ross's wedding because she was afraid of flying. She was also afraid of being away from her friends for so long and didn't want to miss out on any of the fun.