Wales is a bilingual area. This speech situation in linguistics is known as exoglossic.
The principal phonological differences between Welsh English pronunciation standard and RP are the following:
Vowels
1. The distribution of [æ] and [a:] is as in the north of England. Last, dance, chance, etc. tend to have [æ] rather than [a:].
2. Unstressed orthographic «a» tends to be [æ] rather than [ə], e.g.: sofa [so:fæ].
3. There is no contrast between [] and [ə]: rubber [rəbə].
4. [i] at the end is a long vowel: city [siti:].
5. In words like tune, few, used we find [iu] rather than [ju:]: tune [tiun].
6. [ei], [əu] may become monophthongs: bake [bek], boat [bo:t].
7. The vowel as in girl is produced with rounded lips approaching [o:].
8. The vowels [iə], [uə] do not occur in many variants of Welsh English: fear is [fi:jə], poor is [pu:wə].
Consonants
1. Welsh English is non-rhotic, [r] is a tap, or it is also called a flapped [r]. Intrusive and linking [r] do occur.
2. Consonants in intervocalic position, particularly when the preceding vowel is short are doubled: city [sitti].
3. Voiceless plosives tend to be strongly aspirated: in word final position they are generally released and without glottalization, e.g. pit [phith].
4. [1] is clear in all positions.
5. Intonation in Welsh English is very much influenced by the Welsh language.