The EUR leg should be valued in EUR but in a manner consistent with GBP collateral:
- a 3m EURIBOR forward curve consistent with GBP collateral
- a EUR discount curve consistent with GBP collateral
In theory both curves should be jointly bootstrapped to fixed vs EURIBOR swaps collateralized in GBP and EURGBP FX swaps and cross currency swaps collateralized in GBP.
In practice if market data for fixed vs EURIBOR swaps collateralized in GBP is unavailable, you should use fixed vs EURIBOR swaps collateralized in EUR. The theoretical difference between the 3m EURIBOR forward curve under GBP collateral and the 3m EURIBOR forward curve under EUR collateral is a covariance term between EURIBOR and the EUR/GBP basis spread, so assuming that the two curves are the same is equivalent to neglecting the EURIBOR, EUR/GBP basis correlation.
As for the EUR discount curve it should be the EUR OIS adjusted with EUR/GBP OIS basis spread. Beware that the OIS basis spread needs to be bootstrapped: the spread in a zero PV cross currency swap cannot be used directly as it is (roughly) equal to the EUR/GBP OIS basis spread plus the difference between the OIS-Libor spreads in both currencies.
As an alternative if you already have the forward EURGBP FX curve available, you can obtain the adjusted EUR OIS curve as GBP OIS discount x spot EURGBP / forward EURGBP.
Hope this helps.