WordNet is what you want. It's big, containing over a hundred thousand entries, and it's freely available.
However, it's not stored as XML. To access the data, you'll want to use one of the existing WordNet APIs for your language of choice.
Using the APIs is generally pretty straightforward, so I don't think you have to worry much about "learning (a) complex API". For example, borrowing from the WordNet How to for the Python based Natural Language Toolkit (NLTK):
>>> from nltk.corpus import wordnet
>>>
>>> # Get All Synsets for 'dog'
>>> # This is essentially all senses of the word in the db
>>> wordnet.synsets('dog')
[Synset('dog.n.01'), Synset('frump.n.01'), Synset('dog.n.03'),
Synset('cad.n.01'), Synset('frank.n.02'),Synset('pawl.n.01'),
Synset('andiron.n.01'), Synset('chase.v.01')]
>>> # Get the definition and usage for the first synset
>>> wn.synset('dog.n.01').definition
'a member of the genus Canis (probably descended from the common
wolf) that has been domesticated by man since prehistoric times;
occurs in many breeds'
>>> wn.synset('dog.n.01').examples
['the dog barked all night']
>>> # Get antonyms for 'good'
>>> wordnet.synset('good.a.01').lemmas[0].antonyms()
[Lemma('bad.a.01.bad')]
>>> # Get synonyms for the first noun sense of 'dog'
>>> wordnet.synset('dog.n.01').lemmas
[Lemma('dog.n.01.dog'), Lemma('dog.n.01.domestic_dog'),
Lemma('dog.n.01.Canis_familiaris')]
>>> # Get synonyms for all senses of 'dog'
>>> for synset in wordnet.synsets('dog'): print synset.lemmas
[Lemma('dog.n.01.dog'), Lemma('dog.n.01.domestic_dog'),
Lemma('dog.n.01.Canis_familiaris')]
...
[Lemma('frank.n.02.frank'), Lemma('frank.n.02.frankfurter'),
...
While there is an American English bias in WordNet, it supports British spellings and usage. For example, you can look up 'colour' and one of the synsets for 'lift' is 'elevator.n.01'.
Notes on XML
If having the data represented as XML is essential, you could easily use one of the APIs to access the WordNet database
and convert it into XML, e.g. see Thinking XML: Querying WordNet as XML.