...forecast on Wednesday, in what it said was a “much weaker” outlook than before.

实际上，该句有一个很特殊的结构。就是，在谓语动词forcast （预测）后面有两个“状语”，一个时间状语，一个地点状语。

1. 时间状语：on Wednesday （在周三）

2. 地点状语（广义地点或抽象地点）：in what it said was a “much weaker” outlook than before。

（1）what was a “much weaker” outlook than before，是名词性从句作in 的宾语。

（2）it said （该办公室称），作插入语。

词组的意思是：在其所谓的比以往前景差的看法中。

整个句子翻译如下：

Prices will drop 0.3 per cent in 2019, the Office for Budget Responsibility forecast on Wednesday, in what it said was a “much weaker” outlook than before. 

【翻译】英国预算责任办公室周三预测，2019年英国房价将下跌0.3%。该机构曾表示，英国房价前景将比以前“差得多”。

Prices will drop 0.3 per cent in 2019, the Office for Budget Responsibility forecast on Wednesday, in what it said was a “much weaker” outlook than before.

in what it said was a “much weaker” outlook than before做状语修饰forecast，forest这个动作是在办公室说某种比之前更加疲弱的前景中一事时进行的预测。所以译文：并称前景比之前“疲弱得多”不对。应该是先提到了一个主题即：前景比之前“疲弱得多”，之后做出了房价将下跌的预测。

Prices will drop 0.3 per cent in 2019, the Office for Budget Responsibility forecast on Wednesday, in what it said was a “much weaker” outlook than before.

周三，美国预算责任办公室(OfficeforBudgetResponsibility)预测，2019年房价将会下跌0.3%，该办公室称，这一前景比以往“弱得多”。

in what it said ..than before该办公室称，这一前景比以往“弱得多”。