In Britain he had come across a painter______nobody else has heard of.

In Britain he had come across a painter nobody else has heard of.
在英国，他邂逅（偶然遇到）了一个不为人所知的画家。

这里nobody else has heard of为定语从句，修饰和限定句中的painter。

也就是说painter（画家 --- 一个人）是这个定语从句的先行词，那么引导定语从句的关系代词必须可以指代人，即只能填who或that.

又因为关系代词who或that所指代的painter做后面的定语从句中的宾语，所以可以省略。

楼主的句子是下面这样两个句子的整合。
In Britain he had come across a painter.
在英国，他邂逅（偶然遇到）了一个画家。

Nobody else has heard of the painter.（painter为句中的宾语）
再没有其他的人听说过这个画家。

变为含定语从句的主从复合句必然是：
In Britain he had come across a painter （who 或 that 或 / ） nobody else has heard of.

应该是who，which不对
但who what 等都可用that代替，that可省略，所以不填

which指物 who指人

先行词并不是painter
所以不能用who
应该添that而此处省略
------------------------
大概是这样