Client fees and VAT forestalling

We invoice a client, say £1,000 plus VAT.  We charge all the VAT on that invoice but the client pays the balance in installments (say £200 now and another £100 per month for 8 months).

Also some clients pay £100 a month and at the end of each year we knock that off our fees.

How will we be treated for anti-forestalling?  I assume in case 1 we are giving clients over 6 months to pay £300 so they would be caught, but is the rest outside the scope of anti-forestalling?

How about for the second case?  Are the installments treated as including VAT at the relevant rate they are paid?